Small form factor medical sensor structure and method therefor

ABSTRACT

A measurement system for measuring a parameter of the muscular-skeletal system is disclosed. The measurement system comprises a capacitor, a signal generator, a digital counter, counter register, a digital clock, a digital timer, and a data register. The sensor of the measurement system is the capacitor. The measurement system generates a repeating signal having a measurement cycle that corresponds to the capacitance of the capacitor. The capacitor comprises more than one capacitor mechanically in series. Electrically, the capacitor comprises more than one capacitor in parallel. In one embodiment, the capacitor includes a dielectric layer comprising polyimide. A force, pressure, or load is applied to the capacitor that elastically compresses the device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. application Ser. No.12/825,852 filed on Jun. 29, 2010 claiming priority benefit of U.S.Provisional Patent Application No. 61/221,881 filed on Jun. 30, 2009,the entire contents of which are hereby incorporated by reference. Thisapplication further claims the priority benefit of non-provisionalapplication Ser. No. 12/826,349 filed on Jun. 29, 2010 andnon-provisional applications 13/242,277, and 13/242,662 filed on Sep.23, 2011, the entire contents of which are hereby incorporated byreference.

FIELD

The present invention pertains generally to measurement of physicalparameters, and particularly to, but not exclusively, medical electronicdevices for high precision sensing.

BACKGROUND

The skeletal system of a mammal is subject to variations among species.Further changes can occur due to environmental factors, degradationthrough use, and aging. An orthopedic joint of the skeletal systemtypically comprises two or more bones that move in relation to oneanother. Movement is enabled by muscle tissue and tendons attached tothe skeletal system of the joint. Ligaments hold and stabilize the oneor more joint bones positionally. Cartilage is a wear surface thatprevents bone-to-bone contact, distributes load, and lowers friction.

There has been substantial growth in the repair of the human skeletalsystem. In general, orthopedic joints have evolved using informationfrom simulations, mechanical prototypes, and patient data that iscollected and used to initiate improved designs. Similarly, the toolsbeing used for orthopedic surgery have been refined over the years buthave not changed substantially. Thus, the basic procedure forreplacement of an orthopedic joint has been standardized to meet thegeneral needs of a wide distribution of the population. Although thetools, procedure, and artificial joint meet a general need, eachreplacement procedure is subject to significant variation from patientto patient. The correction of these individual variations relies on theskill of the surgeon to adapt and fit the replacement joint using theavailable tools to the specific circumstance.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features of the system are set forth with particularity in theappended claims. The embodiments herein, can be understood by referenceto the following description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates a sensor placed in contact between a femur and atibia for measuring a parameter in accordance with an exampleembodiment;

FIG. 2 illustrates a block diagram of an zero-crossing receiver inaccordance with an example embodiment;

FIG. 3 illustrates a block diagram of the integrated zero-crossingreceiver coupled to a sensing assembly in accordance with an exampleembodiment;

FIG. 4 illustrates a propagation tuned oscillator (PTO) incorporating azero-crossing receiver or an edge detect receiver to maintain positiveclosed-loop feedback in accordance with an example embodiment;

FIG. 5 illustrates a sensor interface incorporating the zero-crossingreceiver in a continuous wave multiplexing arrangement for maintainingpositive closed-loop feedback in accordance with an example embodiment;

FIG. 6 illustrates a block diagram of a propagation tuned oscillator(PTO) incorporating the integrated zero-crossing receiver for operationin continuous wave mode;

FIG. 7 illustrates a sensor interface diagram incorporating theintegrated zero-crossing receiver in a pulse multiplexing arrangementfor maintaining positive closed-loop feedback in accordance with anexample embodiment;

FIG. 8 illustrates a block diagram of a propagation tuned oscillator(PTO) incorporating the integrated zero-crossing receiver for operationin pulse mode in accordance with an example embodiment;

FIG. 9 illustrates a block diagram of an edge-detect receiver circuit inaccordance with an example embodiment;

FIG. 10 illustrates a block diagram of the edge-detect receiver circuitcoupled to a sensing assembly;

FIG. 11 illustrates a sensor interface diagram incorporating theedge-detect receiver circuit in a pulse-echo multiplexing arrangementfor maintaining positive closed-loop feedback in accordance with anexample embodiment;

FIG. 12 illustrates a block diagram of a propagation tuned oscillator(PTO) incorporating the edge-detect receiver circuit for operation inpulse echo mode;

FIG. 13 illustrates a simplified cross-sectional view of a sensingmodule in accordance with an example embodiment;

FIG. 14 illustrates an assemblage for illustrating reflectance andunidirectional modes of operation in accordance with an exampleembodiment;

FIG. 15 illustrates an assemblage that illustrates propagation ofultrasound waves within a waveguide in the bi-directional mode ofoperation of this assemblage;

FIG. 16 illustrates a cross-sectional view of a sensor element toillustrate changes in the propagation of ultrasound waves with changesin the length of a waveguide;

FIG. 17 illustrates a simplified flow chart of method steps for highprecision processing and measurement data in accordance with an exampleembodiment;

FIG. 18 illustrates a block diagram of a medical sensing system inaccordance with an example embodiment;

FIG. 19 illustrates an oscillator configured to generate a measurementcycle corresponding to a capacitor in accordance with an exampleembodiment;

FIG. 20 illustrates a method of force, pressure, or load sensing inaccordance with an example embodiment;

FIG. 21 illustrates a cross-sectional view of a capacitor in accordancewith an example embodiment;

FIG. 22 illustrates the capacitor of FIG. 21 comprising more than onecapacitor coupled mechanically in series in accordance with an exampleembodiment;

FIG. 23 illustrates the capacitor of FIG. 21 comprising more than onecapacitor coupled electrically in parallel in accordance with an exampleembodiment;

FIG. 24 illustrates a top view of a conductive region of the capacitorof FIG. 21 and interconnect thereto in accordance with an exampleembodiment;

FIG. 25 illustrates a cross-sectional view of the interconnect coupledto the capacitor of FIG. 21 in accordance with an example embodiment;

FIG. 26 illustrates a diagram of a method of measuring a force,pressure, or load in accordance with an example embodiment;

FIG. 27 illustrates a medical device having a plurality of sensors inaccordance with an example embodiment;

FIG. 28 illustrates one or more prosthetic components having sensorscoupled to and conforming with non-planar surfaces in accordance with anexample embodiment;

FIG. 29 illustrates a tool having one or more shielded sensors coupledto a non-planar surface in accordance with an example embodiment; and

FIG. 30 illustrates a diagram of a method of using a capacitor as asensor to measure a parameter of the muscular-skeletal system inaccordance with an example embodiment.

DETAILED DESCRIPTION

Embodiments of the invention are broadly directed to measurement ofphysical parameters, and more particularly, to fast-response circuitrythat supports accurate measurement of small sensor changes.

The following description of exemplary embodiment(s) is merelyillustrative in nature and is in no way intended to limit the invention,its application, or uses.

Processes, techniques, apparatus, and materials as known by one ofordinary skill in the art may not be discussed in detail but areintended to be part of the enabling description where appropriate. Forexample specific computer code may not be listed for achieving each ofthe steps discussed, however one of ordinary skill would be able,without undo experimentation, to write such code given the enablingdisclosure herein. Such code is intended to fall within the scope of atleast one exemplary embodiment.

In all of the examples illustrated and discussed herein, any specificmaterials, such as temperatures, times, energies, and materialproperties for process steps or specific structure implementationsshould be interpreted to be illustrative only and non-limiting.Processes, techniques, apparatus, and materials as known by one ofordinary skill in the art may not be discussed in detail but areintended to be part of an enabling description where appropriate. Itshould also be noted that the word “coupled” used herein implies thatelements may be directly coupled together or may be coupled through oneor more intervening elements.

Additionally, the sizes of structures used in exemplary embodiments arenot limited by any discussion herein (e.g., the sizes of structures canbe macro (centimeter, meter, and larger sizes), micro (micrometer), andnanometer size and smaller).

Notice that similar reference numerals and letters refer to similaritems in the following figures, and thus once an item is defined in onefigure, it may not be discussed or further defined in the followingfigures.

In a first embodiment, an ultrasonic measurement system comprises one ormore ultrasonic transducers, an ultrasonic waveguide, and a propagationtuned oscillator (PTO) or Phase Locked Loop (PLL). The ultrasonicmeasurement system in this embodiment employs a continuous mode (CM) ofoperation to evaluate propagation characteristics of continuousultrasonic waves in the waveguide by way of closed-loop feedback todetermine levels of applied forces on the waveguide.

In a second embodiment, an ultrasonic measurement system comprises oneor more ultrasonic transducers, an ultrasonic waveguide, and apropagation tuned oscillator (PTO) or Phase Locked Loop (PLL). Theultrasonic measurement system in this embodiment employs a pulse mode(PM) of operation to evaluate propagation characteristics of pulsedultrasonic waves in the waveguide by way of closed-loop feedback todetermine levels of applied forces on the waveguide.

In a third embodiment, an ultrasonic measurement system comprises one ormore ultrasonic transducers, an ultrasonic waveguide, and a propagationtuned oscillator (PTO) or Phase Locked Loop (PLL). The ultrasonicmeasurement system in this embodiment employs a pulse echo mode (PE) ofoperation to evaluate propagation characteristics of ultrasonic echoreflections in the waveguide by way of closed-loop feedback to determinelevels of applied forces on the waveguide.

FIG. 1 is an illustration of a sensor 100 placed in contact between afemur 102 and a tibia 108 for measuring a parameter in accordance withan exemplary embodiment. In general, a sensor 100 is placed in contactwith or in proximity to the muscular-skeletal system to measure aparameter. In a non-limiting example, sensor 100 is used to measure aparameter of a muscular-skeletal system during a procedure such as aninstallation of an artificial joint. Embodiments of sensor 100 arebroadly directed to measurement of physical parameters, and moreparticularly, to evaluating changes in the transit time of a pulsedenergy wave propagating through a medium. In-situ measurements duringorthopedic joint implant surgery would be of substantial benefit toverify an implant is in balance and under appropriate loading ortension. In one embodiment, the instrument is similar to and operatesfamiliarly with other instruments currently used by surgeons. This willincrease acceptance and reduce the adoption cycle for a new technology.The measurements will allow the surgeon to ensure that the implantedcomponents are installed within predetermined ranges that maximize theworking life of the joint prosthesis and reduce costly revisions.Providing quantitative measurement and assessment of the procedure usingreal-time data will produce results that are more consistent. A furtherissue is that there is little or no implant data generated from theimplant surgery, post-operatively, and long term. Sensor 100 can provideimplant status data to the orthopedic manufacturers and surgeons.Moreover, data generated by direct measurement of the implanted jointitself would greatly improve the knowledge of implanted joint operationand joint wear thereby leading to improved design and materials.

In at least one exemplary embodiment, an energy pulse is directed withinone or more waveguides in sensor 100 by way of pulse mode operations andpulse shaping. The waveguide is a conduit that directs the energy pulsein a predetermined direction. The energy pulse is typically confinedwithin the waveguide. In one embodiment, the waveguide comprises apolymer material. For example, urethane or polyethylene are polymerssuitable for forming a waveguide. The polymer waveguide can becompressed and has little or no hysteresis in the system. Alternatively,the energy pulse can be directed through the muscular-skeletal system.In one embodiment, the energy pulse is directed through bone of themuscular-skeletal system to measure bone density. A transit time of anenergy pulse is related to the material properties of a medium throughwhich it traverses. This relationship is used to generate accuratemeasurements of parameters such as distance, weight, strain, pressure,wear, vibration, viscosity, and density to name but a few.

Sensor 100 can be size constrained by form factor requirements offitting within a region the muscular-skeletal system or a component suchas a tool, equipment, or artificial joint. In a non-limiting example,sensor 100 is used to measure load and balance of an installedartificial knee joint. A knee prosthesis comprises a femoral prostheticcomponent 104, an insert, and a tibial prosthetic component 106. Adistal end of femur 102 is prepared and receives femoral prostheticcomponent 104. Femoral prosthetic component 104 typically has twocondyle surfaces that mimic a natural femur. As shown, femoralprosthetic component 104 has single condyle surface being coupled tofemur 102. Femoral prosthetic component 104 is typically made of a metalor metal alloy.

A proximal end of tibia 108 is prepared to receive tibial prostheticcomponent 106. Tibial prosthetic component 106 is a support structurethat is fastened to the proximal end of the tibia and is usually made ofa metal or metal alloy. The tibial prosthetic component 106 also retainsthe insert in a fixed position with respect to tibia 108. The insert isfitted between femoral prosthetic component 104 and tibial prostheticcomponent 106. The insert has at least one bearing surface that is incontact with at least condyle surface of femoral prosthetic component104. The condyle surface can move in relation to the bearing surface ofthe insert such that the lower leg can rotate under load. The insert istypically made of a high wear plastic material that minimizes friction.

In a knee joint replacement process, the surgeon affixes femoralprosthetic component 104 to the femur 102 and tibial prostheticcomponent 106 to tibia 108. The tibial prosthetic component 106 caninclude a tray or plate affixed to the planarized proximal end of thetibia 108. Sensor 100 is placed between a condyle surface of femoralprosthetic component 104 and a major surface of tibial prostheticcomponent 106. The condyle surface contacts a major surface of sensor100. The major surface of sensor 100 approximates a surface of theinsert. Tibial prosthetic component 106 can include a cavity or tray onthe major surface that receives and retains sensor 100 during ameasurement process. Tibial prosthetic component 106 and sensor 100 hasa combined thickness that represents a combined thickness of tibialprosthetic component 106 and a final (or chronic) insert of the kneejoint.

In one embodiment, two sensors 100 are fitted into two separatecavities, the cavities are within a trial insert (that may also bereferred to as the tibial insert, rather than the tibial componentitself) that is held in position by tibial component 106. One or twosensors 100 may be inserted between femoral prosthetic component 104 andtibial prosthetic component 106. Each sensor is independent and eachmeasures a respective condyle of femur 102. Separate sensors alsoaccommodate a situation where a single condyle is repaired and only asingle sensor is used. Alternatively, the electronics can be sharedbetween two sensors to lower cost and complexity of the system. Theshared electronics can multiplex between each sensor module to takemeasurements when appropriate. Measurements taken by sensor 100 aid thesurgeon in modifying the absolute loading on each condyle and thebalance between condyles. Although shown for a knee implant, sensor 100can be used to measure other orthopedic joints such as the spine, hip,shoulder, elbow, ankle, wrist, interphalangeal joint,metatarsophalangeal joint, metacarpophalangeal joints, and others.Alternatively, sensor 100 can also be adapted to orthopedic tools toprovide measurements.

The prosthesis incorporating sensor 100 emulates the function of anatural knee joint. Sensor 100 can measure loads or other parameters atvarious points throughout the range of motion. Data from sensor 100 istransmitted to a receiving station 110 via wired or wirelesscommunications. In a first embodiment, sensor 100 is a disposablesystem. Sensor 100 can be disposed of after using sensor 100 tooptimally fit the joint implant. Sensor 100 is a low cost disposablesystem that reduces capital costs, operating costs, facilitates rapidadoption of quantitative measurement, and initiates evidentiary basedorthopedic medicine. In a second embodiment, a methodology can be put inplace to clean and sterilize sensor 100 for reuse. In a thirdembodiment, sensor 100 can be incorporated in a tool instead of being acomponent of the replacement joint. The tool can be disposable or becleaned and sterilized for reuse. In a fourth embodiment, sensor 100 canbe a permanent component of the replacement joint. Sensor 100 can beused to provide both short term and long term post-operative data on theimplanted joint. In a fifth embodiment, sensor 100 can be coupled to themuscular-skeletal system. In all of the embodiments, receiving station110 can include data processing, storage, or display, or combinationthereof and provide real time graphical representation of the level anddistribution of the load. Receiving station 110 can record and provideaccounting information of sensor 100 to an appropriate authority.

In an intra-operative example, sensor 100 can measure forces (Fx, Fy,Fz) with corresponding locations and torques (e.g. Tx, Ty, and Tz) onthe femoral prosthetic component 104 and the tibial prosthetic component106. The measured force and torque data is transmitted to receivingstation 110 to provide real-time visualization for assisting the surgeonin identifying any adjustments needed to achieve optimal joint pressureand balancing. The data has substantial value in determining ranges ofload and alignment tolerances required to minimize rework and maximizepatient function and longevity of the joint.

As mentioned previously, sensor 100 can be used for other jointsurgeries; it is not limited to knee replacement implant or implants.Moreover, sensor 100 is not limited to trial measurements. Sensor 100can be incorporated into the final joint system to provide datapost-operatively to determine if the implanted joint is functioningcorrectly. Early determination of a problem using sensor 100 can reducecatastrophic failure of the joint by bringing awareness to a problemthat the patient cannot detect. The problem can often be rectified witha minimal invasive procedure at lower cost and stress to the patient.Similarly, longer term monitoring of the joint can determine wear ormisalignment that if detected early can be adjusted for optimal life orreplacement of a wear surface with minimal surgery thereby extending thelife of the implant. In general, sensor 100 can be shaped such that itcan be placed or engaged or affixed to or within load bearing surfacesused in many orthopedic applications (or used in any orthopedicapplication) related to the musculoskeletal system, joints, and toolsassociated therewith. Sensor 100 can provide information on acombination of one or more performance parameters of interest such aswear, stress, kinematics, kinetics, fixation strength, ligament balance,anatomical fit and balance.

FIG. 2 is a block diagram of a zero-crossing receiver 200 in accordancewith one embodiment. In a first embodiment, the zero-crossing receiver200 is provided to detect transition states of energy waves, such as thetransition of each energy wave through a mid-point of a symmetrical orcyclical waveform. This enables capturing of parameters including, butnot limited to, transit time, phase, or frequency of the energy waves.The receiver rapidly responds to a signal transition and outputs adigital pulse that is consistent with the energy wave transitioncharacteristics and with minimal delay. The zero-crossing receiver 200further discriminates between noise and the energy waves of interest,including very low level waves by way of adjustable levels of noisereduction. A noise reduction section 218 comprises a filtering stage andan offset adjustment stage to perform noise suppression accurately overa wide range of amplitudes including low level waves.

In a second embodiment, a zero-crossing receiver is provided to convertan incoming symmetrical, cyclical, or sine wave to a square orrectangular digital pulse sequence with superior performance for verylow level input signals. The digital pulse sequence represents pulsetiming intervals that are consistent with the energy wave transitiontimes. The zero-crossing receiver is coupled with a sensing assembly togenerate the digital pulse sequence responsive to evaluating transitionsof the incoming sine wave. This digital pulse sequence conveys timinginformation related to parameters of interest, such as applied forces,associated with the physical changes in the sensing assembly.

In a third embodiment, the integrated zero-crossing receiver isincorporated within a propagation tuned oscillator (PTO) to maintainpositive closed-loop feedback when operating in a continuous wave modeor pulse-loop mode. The integrated edge zero-crossing receiver iselectrically integrated with the PTO by multiplexing input and outputcircuitry to achieve ultra low-power and small compact size. Electricalcomponents of the PTO are integrated with components of thezero-crossing receiver to assure adequate sensitivity to low-levelsignals.

In one embodiment, low power zero-crossing receiver 200 can beintegrated with other circuitry of the propagation tuned oscillator tofurther improve performance at low signal levels. The zero-crossingreceiver 200 comprises a preamplifier 206, a filter 208, an offsetadjustment circuitry 210, a comparator 212, and a digital pulse circuit214. The filter 208 and offset adjustment circuitry 210 constitute anoise reduction section 218 as will be explained ahead. Thezero-crossing receiver 200 can be implemented in discrete analogcomponents, digital components or combination thereof. The integratedzero-crossing receiver 200 practices measurement methods that detect themidpoint of energy waves at specified locations, and under specifiedconditions, to enable capturing parameters including, but not limitedto, transit time, phase, or frequency of energy waves. A briefdescription of the method of operation is as follows.

An incoming energy wave 202 is coupled from an electrical connection,antenna, or transducer to an input 204 of zero-crossing receiver 200.Input 204 of zero-crossing receiver 200 is coupled to pre-amplifier 206to amplify the incoming energy wave 202. The amplified signal isfiltered by filter 208. Filter 208 is coupled to an output ofpre-amplifier 206 and an input of offset adjustment circuitry 210. Inone configuration, filter 208 is a low-pass filter to remove highfrequency components above the incoming energy wave 202 bandwidth. Inanother arrangement, the filter is a band-pass filter with a pass-bandcorresponding to the bandwidth of the incoming energy wave 202. It isnot however limited to either arrangement. The offset of the filteredamplified wave is adjusted by offset adjustment circuitry 210. An inputof comparator 212 is coupled to an output of offset adjustment circuitry210. Comparator 212 monitors the amplified waveforms and triggersdigital pulse circuitry 214 whenever the preset trigger level isdetected. Digital pulse circuit 214 has an input coupled to the outputof comparator 212 and an output for providing digital pulse 216. Thedigital pulse 216 can be further coupled to signal processing circuitry,as will be explained ahead.

In at least one embodiment, the electronic components are operativelycoupled together as blocks of integrated circuits. As will be shownahead, this integrated arrangement performs its specific functionsefficiently with a minimum number of components. This is because thecircuit components are partitioned between structures within anintegrated circuit and discrete components, as well as innovativepartitioning of analog and digital functions, to achieve the requiredperformance with a minimum number of components and minimum powerconsumption.

FIG. 3 illustrates a block diagram of the integrated zero-crossingreceiver 200 coupled to a sensing assembly 300 in accordance with anexemplary embodiment. The pre-amplifier 206 and the digital pulsecircuit 214 are shown for reference and discussion. In one embodiment,sensing assembly 300 comprises a transmitter transducer 302, an energypropagating structure (or medium) 304, and a receiver transducer 306. Aswill be explained further hereinbelow, the sensing assembly 300 in oneembodiment is part of a sensory device that measures a parameter such asforce, pressure, or load. In a non-limiting example, an externalparameter such as an applied force 308 affects the sensing assembly 200.As shown, applied force 308 modifies propagating structure 304dimensionally. In general, the sensing assembly 300 conveys one or moreparameters of interest such as distance, force, weight, strain,pressure, wear, vibration, viscosity, density, direction, anddisplacement related to a change in energy propagating structure 304. Anexample is measuring loading applied by a joint of the muscular-skeletalsystem as disclosed above using sensing assembly 300 between the bonesof the joint.

A transducer driver circuit (not shown) drives the transmittertransducer 302 of the sensing assembly 300 to produce energy waves 310that are directed into the energy propagating structure 304. Changes inthe energy propagating medium 304 due to an applied parameter such asapplied forces 308 change the frequency, phase, and transit time ofenergy waves 310 (or pulses). In one embodiment, applied forces 308affect the length of propagating structure 304 in a direction of a pathof propagation of energy waves 310. The zero-crossing receiver 200 iscoupled to the receiver transducer 306 to detect zero-crossings of thereproduced energy wave 202. Upon detecting a zero-crossing digital pulsecircuit 214 is triggered to output a pulse 216. The timing of thedigital pulse 216 conveys the parameters of interest (e.g., distance,force weight, strain, pressure, wear, vibration, viscosity, density,direction, displacement, etc.).

Measurement methods that rely on such propagation of energy waves 310 orpulses of energy waves are required to achieve highly accurate andcontrolled detection of energy waves or pulses. Moreover, pulses ofenergy waves may contain multiple energy waves with complex waveformstherein leading to potential ambiguity of detection. In particular,directing energy waves 310 into the energy propagating structure 304 cangenerate interference patterns caused by nulls and resonances of thewaveguide, as well as characteristics of the generated energy waves 310.These interference patterns can multiply excited waveforms that resultin distortion of the edges of the original energy wave.

Briefly referring back to FIG. 2, to reliably detect the arrival of apulse of energy waves, the zero-crossing receiver 200 leverages noisereduction section 218 that incorporates two forms of noise reduction.Frequencies above the operating frequencies for physical measurements ofthe parameters of interest are attenuated with the filter 208. Inaddition, the offset level of the incoming waveform is adjusted by theoffset adjustment 210 to optimize the voltage level at which thecomparator 212 triggers an output pulse. This is more reliable thanamplifying the incoming waveform because it does not add additionalamplification of noise present on the input. The combination of rapidresponse to the arrival of incoming symmetrical, cyclical, or sine waveswith adjustable levels of noise reduction achieves reliablezero-crossing detection by way of the ultra low power zero-crossingreceiver 200 with superior performance for very low level signals.

There are a wide range of applications for compact measurement modulesor devices having ultra low power circuitry that enables the design andconstruction of highly performing measurement modules or devices thatcan be tailored to fit a wide range of nonmedical and medicalapplications. Applications for highly compact measurement modules ordevices may include, but are not limited to, disposable modules ordevices as well as reusable modules or devices and modules or devicesfor long term use. In addition to nonmedical applications, examples of awide range of potential medical applications may include, but are notlimited to, implantable devices, modules within implantable devices,intra-operative implants or modules within intra-operative implants ortrial inserts, modules within inserted or ingested devices, moduleswithin wearable devices, modules within handheld devices, modules withininstruments, appliances, equipment, or accessories of all of these, ordisposables within implants, trial inserts, inserted or ingesteddevices, wearable devices, handheld devices, instruments, appliances,equipment, or accessories to these devices, instruments, appliances, orequipment.

FIG. 4 is an exemplary block diagram 400 of a propagation tunedoscillator (PTO) 404 to maintain positive closed-loop feedback inaccordance with an exemplary embodiment. The measurement system includesa sensing assemblage 401 and propagation tuned oscillator (PTO) 404 thatdetects energy waves 402 in one or more waveguides 403 of the sensingassemblage 401. In one embodiment, energy waves 402 are ultrasoundwaves. A pulse 411 is generated in response to the detection of energywaves 402 to initiate a propagation of a new energy wave in waveguide403. It should be noted that ultrasound energy pulses or waves, theemission of ultrasound pulses or waves by ultrasound resonators ortransducers, transmitted through ultrasound waveguides, and detected byultrasound resonators or transducers are used merely as examples ofenergy pulses, waves, and propagation structures and media. Otherembodiments herein contemplated can utilize other wave forms, such as,light.

The sensing assemblage 401 comprises transducer 405, transducer 406, anda waveguide 403 (or energy propagating structure). In a non-limitingexample, sensing assemblage 401 is affixed to load bearing or contactingsurfaces 408. External forces applied to the contacting surfaces 408compress the waveguide 403 and change the length of the waveguide 403.Under compression, transducers 405 and 406 will also be move closertogether. The change in distance affects the transit time 407 of energywaves 402 transmitted and received between transducers 405 and 406. Thepropagation tuned oscillator 404 in response to these physical changeswill detect each energy wave sooner (e.g. shorter transit time) andinitiate the propagation of new energy waves associated with the shortertransit time. As will be explained below, this is accomplished by way ofPTO 404 in conjunction with the pulse generator 410, the mode control412, and the phase detector 414.

Notably, changes in the waveguide 403 (energy propagating structure orstructures) alter the propagation properties of the medium ofpropagation (e.g. transit time 407). The energy wave can be a continuouswave or a pulsed energy wave. A pulsed energy wave approach reducespower dissipation allowing for a temporary power source such as abattery or capacitor to power the system during the course of operation.In at least one exemplary embodiment, a continuous wave energy wave or apulsed energy wave is provided by transducer 405 to a first surface ofwaveguide 403. Transducer 405 generates energy waves 402 that arecoupled into waveguide 403. In a non-limiting example, transducer 405 isa piezo-electric device capable of transmitting and receiving acousticsignals in the ultrasonic frequency range.

Transducer 406 is coupled to a second surface of waveguide 403 toreceive the propagated pulsed signal and generates a correspondingelectrical signal. The electrical signal output by transducer 406 iscoupled to phase detector 414. In general, phase detector 414 is adetection circuit that compares the timing of a selected point on thewaveform of the detected energy wave with respect to the timing of thesame point on the waveform of other propagated energy waves. In a firstembodiment, phase detector 414 can be a zero-crossing receiver. In asecond embodiment, phase detector 414 can be an edge-detect receiver. Ina third embodiment, phase detector 414 can be a phase locked loop. Inthe example where sensing assemblage 401 is compressed, the detection ofthe propagated energy waves 402 occurs earlier (due to thelength/distance reduction of waveguide 403) than a signal prior toexternal forces being applied to contacting surfaces. Pulse generator410 generates a new pulse in response to detection of the propagatedenergy waves 402 by phase detector 414. The new pulse is provided totransducer 405 to initiate a new energy wave sequence. Thus, each energywave sequence is an individual event of energy wave propagation, energywave detection, and energy wave emission that maintains energy waves 402propagating in waveguide 403.

The transit time 407 of a propagated energy wave is the time it takes anenergy wave to propagate from the first surface of waveguide 403 to thesecond surface. There is delay associated with each circuit describedabove. Typically, the total delay of the circuitry is significantly lessthan the propagation time of an energy wave through waveguide 403. Inaddition, under equilibrium conditions variations in circuit delay areminimal. Multiple pulse to pulse timings can be used to generate anaverage time period when change in external forces occur relativelyslowly in relation to the pulsed signal propagation time such as in aphysiologic or mechanical system. The digital counter 420 in conjunctionwith electronic components counts the number of propagated energy wavesto determine a corresponding change in the length of the waveguide 403.These changes in length change in direct proportion to the externalforce thus enabling the conversion of changes in parameter or parametersof interest into electrical signals.

The block diagram 400 further includes counting and timing circuitry.More specifically, the timing, counting, and clock circuitry comprises adigital timer 420, a digital timer 422, a digital clock 426, and a dataregister 424. The digital clock 426 provides a clock signal to digitalcounter 420 and digital timer 422 during a measurement sequence. Thedigital counter 420 is coupled to the propagation tuned oscillator 404.Digital timer 422 is coupled to data register 424. Digital timer 420,digital timer, 422, digital clock 426 and data register 424 capturetransit time 407 of energy waves 402 emitted by ultrasound resonator ortransducer 405, propagated through waveguide 403, and detected by orultrasound resonator or transducer 405 or 406 depending on the mode ofthe measurement of the physical parameters of interest applied tosurfaces 408. The operation of the timing and counting circuitry isdisclosed in more detail hereinbelow.

The measurement data can be analyzed to achieve accurate, repeatable,high precision and high resolution measurements. This method enables thesetting of the level of precision or resolution of captured data tooptimize trade-offs between measurement resolution versus frequency,including the bandwidth of the sensing and data processing operations,thus enabling a sensing module or device to operate at its optimaloperating point without compromising resolution of the measurements.This is achieved by the accumulation of multiple cycles of excitationand transit time instead of averaging transit time of multipleindividual excitation and transit cycles. The result is accurate,repeatable, high precision and high resolution measurements ofparameters of interest in physical systems.

In at least one exemplary embodiment, propagation tuned oscillator 404in conjunction with one or more sensing assemblages 401 are used to takemeasurements on a muscular-skeletal system. In a non-limiting example,sensing assemblage 401 is placed between a femoral prosthetic componentand tibial prosthetic component to provide measured load informationthat aids in the installation of an artificial knee joint. Sensingassemblage 401 can also be a permanent component or a muscular-skeletaljoint or artificial muscular-skeletal joint to monitor joint function.The measurements can be made in extension and in flexion. In theexample, assemblage 401 is used to measure the condyle loading todetermine if it falls within a predetermined range and location. Basedon the measurement, the surgeon can select the thickness of the insertsuch that the measured loading and incidence with the final insert inplace will fall within the predetermined range. Soft tissue tensioningcan be used by a surgeon to further optimize the force or pressure.Similarly, two assemblages 401 can be used to measure both condylessimultaneously or multiplexed. The difference in loading (e.g. balance)between condyles can be measured. Soft tissue tensioning can be used toreduce the force on the condyle having the higher measured loading toreduce the measured pressure difference between condyles.

One method of operation holds the number of energy waves propagatingthrough waveguide 403 as a constant integer number. A time period of anenergy wave corresponds to energy wave periodicity. A stable time periodis one in which the time period changes very little over a number ofenergy waves. This occurs when conditions that affect sensing assemblage401 stay consistent or constant. Holding the number of energy wavespropagating through waveguide 403 to an integer number is a constraintthat forces a change in the time between pulses when the length ofwaveguide 403 changes. The resulting change in time period of eachenergy wave corresponds to a change in aggregate energy wave time periodthat is captured using digital counter 420 as a measurement of changesin external forces or conditions applied to contacting surfaces 408.

A further method of operation according to one embodiment is describedhereinbelow for energy waves 402 propagating from transducer 405 andreceived by transducer 406. In at least one exemplary embodiment, energywaves 402 are an ultrasonic energy wave. Transducers 405 and 406 arepiezo-electric resonator transducers. Although not described, wavepropagation can occur in the opposite direction being initiated bytransducer 406 and received by transducer 405. Furthermore, detectingultrasound resonator transducer 406 can be a separate ultrasoundresonator as shown or transducer 405 can be used solely depending on theselected mode of propagation (e.g. reflective sensing). Changes inexternal forces or conditions applied to contacting surfaces 408 affectthe propagation characteristics of waveguide 403 and alter transit time407. As mentioned previously, propagation tuned oscillator 404 holdsconstant an integer number of energy waves 402 propagating throughwaveguide 403 (e.g. an integer number of pulsed energy wave timeperiods) thereby controlling the repetition rate. As noted above, oncePTO 404 stabilizes, the digital counter 420 digitizes the repetitionrate of pulsed energy waves, for example, by way of edge-detection, aswill be explained hereinbelow in more detail.

In an alternate embodiment, the repetition rate of pulsed energy waves402 emitted by transducer 405 can be controlled by pulse generator 410.The operation remains similar where the parameter to be measuredcorresponds to the measurement of the transit time 407 of pulsed energywaves 402 within waveguide 403. It should be noted that an individualultrasonic pulse can comprise one or more energy waves with a dampingwave shape. The energy wave shape is determined by the electrical andmechanical parameters of pulse generator 410, interface material ormaterials, where required, and ultrasound resonator or transducer 405.The frequency of the energy waves within individual pulses is determinedby the response of the emitting ultrasound resonator 404 to excitationby an electrical pulse 411. The mode of the propagation of the pulsedenergy waves 402 through waveguide 403 is controlled by mode controlcircuitry 412 (e.g., reflectance or uni-directional). The detectingultrasound resonator or transducer may either be a separate ultrasoundresonator or transducer 406 or the emitting resonator or transducer 405depending on the selected mode of propagation (reflectance orunidirectional).

In general, accurate measurement of physical parameters is achieved atan equilibrium point having the property that an integer number ofpulses are propagating through the energy propagating structure at anypoint in time. Measurement of changes in the “time-of-flight” or transittime of ultrasound energy waves within a waveguide of known length canbe achieved by modulating the repetition rate of the ultrasound energywaves as a function of changes in distance or velocity through themedium of propagation, or a combination of changes in distance andvelocity, caused by changes in the parameter or parameters of interest.

Measurement methods that rely on the propagation of energy waves, orenergy waves within energy pulses, may require the detection of aspecific point of energy waves at specified locations, or underspecified conditions, to enable capturing parameters including, but notlimited to, transit time, phase, or frequency of the energy waves.Measurement of the changes in the physical length of individualultrasound waveguides may be made in several modes. Each assemblage ofone or two ultrasound resonators or transducers combined with anultrasound waveguide may be controlled to operate in six differentmodes. This includes two wave shape modes: continuous wave or pulsedwaves, and three propagation modes: reflectance, unidirectional, andbi-directional propagation of the ultrasound wave. The resolution ofthese measurements can be further enhanced by advanced processing of themeasurement data to enable optimization of the trade-offs betweenmeasurement resolution versus length of the waveguide, frequency of theultrasound waves, and the bandwidth of the sensing and data captureoperations, thus achieving an optimal operating point for a sensingmodule or device.

Measurement by propagation tuned oscillator 404 and sensing assemblage401 enables high sensitivity and high signal-to-noise ratio. Thetime-based measurements are largely insensitive to most sources of errorthat may influence voltage or current driven sensing methods anddevices. The resulting changes in the transit time of operationcorrespond to frequency, which can be measured rapidly, and with highresolution. This achieves the required measurement accuracy andprecision thus capturing changes in the physical parameters of interestand enabling analysis of their dynamic and static behavior.

These measurements may be implemented with an integrated wirelesssensing module or device having an encapsulating structure that supportssensors and load bearing or contacting surfaces and an electronicassemblage that integrates a power supply, sensing elements, energytransducer or transducers and elastic energy propagating structure orstructures, biasing spring or springs or other form of elastic members,an accelerometer, antennas and electronic circuitry that processesmeasurement data as well as controls all operations of ultrasoundgeneration, propagation, and detection and wireless communications. Theelectronics assemblage also supports testability and calibrationfeatures that assure the quality, accuracy, and reliability of thecompleted wireless sensing module or device.

The level of accuracy and resolution achieved by the integration ofenergy transducers and an energy propagating structure or structurescoupled with the electronic components of the propagation tunedoscillator enables the construction of, but is not limited to, compactultra low power modules or devices for monitoring or measuring theparameters of interest. The flexibility to construct sensing modules ordevices over a wide range of sizes enables sensing modules to betailored to fit a wide range of applications such that the sensingmodule or device may be engaged with, or placed, attached, or affixedto, on, or within a body, instrument, appliance, vehicle, equipment, orother physical system and monitor or collect data on physical parametersof interest without disturbing the operation of the body, instrument,appliance, vehicle, equipment, or physical system.

Referring to FIG. 17, a simplified flow chart 1700 of method steps forhigh precision processing and measurement data is shown in accordancewith an exemplary embodiment. The method 1700 can be practiced with moreor less than the steps shown, and is not limited to the order of stepsshown. The method steps correspond to FIG. 4 to be practiced with theaforementioned components or any other components suitable for suchprocessing, for example, electrical circuitry to control the emission ofenergy pulses or waves and to capture the repetition rate of the energypulses or frequency of the energy waves propagating through the elasticenergy propagating structure or medium.

In a step 1702, the process initiates a measurement operation. In a step1704, a known state is established by resetting digital timer 422 anddata register 424. In a step 1706, digital counter 420 is preset to thenumber of measurement cycles over which measurements will be taken andcollected. In a step 1708, the measurement cycle is initiated and aclock output of digital clock 426 is enabled. A clock signal fromdigital clock 426 is provided to both digital counter 420 and digitaltimer 422. An elapsed time is counted by digital timer 420 based on thefrequency of the clock signal output by digital clock 426. In a step1710, digital timer 422 begins tracking the elapsed time.Simultaneously, digital counter 420 starts decrementing a count aftereach measurement sequence. In one embodiment, digital counter 420 isdecremented as each energy wave propagates through waveguide 403 and isdetected by transducer 406. Digital counter 420 counts down until thepreset number of measurement cycles has been completed. In a step 1712,energy wave propagation is sustained by propagation tuned oscillator404, as digital counter 420 is decremented by the detection of apropagated energy wave. In a step 1714, energy wave detection, emission,and propagation continue while the count in digital counter 420 isgreater than zero. In a step 1716, the clock input of digital timer 422is disabled upon reaching a zero count on digital counter 420 thuspreventing digital counter 420 and digital timer 422 from being clocked.In one embodiment, the preset number of measurement cycles provided todigital counter 420 is divided by the elapsed time measured by digitaltimer 422 to calculate a frequency of propagated energy waves.Conversely, the number can be calculated as a transit time by dividingthe elapsed time from digital timer 422 by the preset number ofmeasurement cycles. Finally, in a step 1718, the resulting value istransferred to register 424. The number in data register 424 can bewirelessly transmitted to a display and database. The data from dataregister 424 can be correlated to a parameter being measured. Theparameter such as a force or load is applied to the propagation medium(e.g. waveguide 403) such that parameter changes also change thefrequency or transit time calculation of the measurement. A relationshipbetween the material characteristics of the propagation medium and theparameter is used with the measurement value (e.g. frequency, transittime, phase) to calculate a parameter value.

The method 1700 practiced by the example assemblage of FIG. 4, and byway of the digital counter 420, digital timer 422, digital clock 426 andassociated electronic circuitry analyzes the digitized measurement dataaccording to operating point conditions. In particular, these componentsaccumulate multiple digitized data values to improve the level ofresolution of measurement of changes in length or other aspect of anelastic energy propagating structure or medium that can alter thetransit time of energy pulses or waves propagating within the elasticenergy propagating structure or medium. The digitized data is summed bycontrolling the digital counter 420 to run through multiple measurementcycles, each cycle having excitation and transit phases such that thereis not lag between successive measurement cycles, and capturing thetotal elapsed time. The counter is sized to count the total elapsed timeof as many measurement cycles as required to achieve the requiredresolution without overflowing its accumulation capacity and withoutcompromising the resolution of the least significant bit of the counter.The digitized measurement of the total elapsed transit time issubsequently divided by the number of measurement cycles to estimate thetime of the individual measurement cycles and thus the transit time ofindividual cycles of excitation, propagation through the elastic energypropagating structure or medium, and detection of energy pulses orwaves. Accurate estimates of changes in the transit time of the energypulses or waves through the elastic energy propagating structure ormedium are captured as elapsed times for excitation and detection of theenergy pulses or waves are fixed.

Summing individual measurements before dividing to estimate the averagemeasurement value data values produces superior results to averaging thesame number of samples. The resolution of count data collected from adigital counter is limited by the resolution of theleast-significant-bit in the counter. Capturing a series of counts andaveraging them does not produce greater precision than thisleast-significant-bit, that is the precision of a single count.Averaging does reduce the randomness of the final estimate if there israndom variation between individual measurements. Summing the counts ofa large number of measurement cycles to obtain a cumulative count thencalculating the average over the entire measurement period improves theprecision of the measurement by interpolating the component of themeasurement that is less than the least significant bit of the counter.The precision gained by this procedure is on the order of the resolutionof the least-significant-bit of the counter divided by the number ofmeasurement cycles summed.

The size of the digital counter and the number of measurement cyclesaccumulated may be greater than the required level of resolution. Thisnot only assures performance that achieves the level of resolutionrequired, but also averages random component within individual countsproducing highly repeatable measurements that reliably meet the requiredlevel of resolution.

The number of measurement cycles is greater than the required level ofresolution. This not only assures performance that achieves the level ofresolution required, but also averages any random component withinindividual counts producing highly repeatable measurements that reliablymeet the required level of resolution.

FIG. 5 is a sensor interface diagram incorporating the zero-crossingreceiver 200 in a continuous wave multiplexing arrangement formaintaining positive closed-loop feedback in accordance with oneembodiment. The positive closed-loop feedback is illustrated by the boldline path. Initially, multiplexer (mux) 502 receives as input a clocksignal 504, which is passed to the transducer driver 506 to produce thedrive line signal 508. Analog multiplexer (mux) 510 receives drive linesignal 508, which is passed to the transmitter transducer 512 togenerate energy waves 514. Transducer 512 is located at a first locationof an energy propagating medium. The emitted energy waves 514 propagatethrough the energy propagating medium. Receiver transducer 516 islocated at a second location of the energy propagating medium. Receivertransducer 516 captures the energy waves 514, which are fed to analogmux 520 and passed to the zero-crossing receiver 200. The capturedenergy waves by transducer 516 are indicated by electrical waves 518provided to mux 520. Zero-crossing receiver 200 outputs a pulsecorresponding to each zero crossing detected from captured electricalwaves 518. The zero crossings are counted and used to determine changesin the phase and frequency of the energy waves propagating through theenergy propagating medium. In a non-limiting example, a parameter suchas applied force is measured by relating the measured phase andfrequency to a known relationship between the parameter (e.g. force) andthe material properties of the energy propagating medium. In general,pulse sequence 522 corresponds to the detected signal frequency. Thezero-crossing receiver 200 is in a feedback path of the propagationtuned oscillator. The pulse sequence 522 is coupled through mux 502 in apositive closed-loop feedback path. The pulse sequence 522 disables theclock signal 504 such that the path providing pulse sequence 522 iscoupled to driver 506 to continue emission of energy waves into theenergy propagating medium and the path of clock signal 504 to driver 506is disabled.

FIG. 6 is an exemplary block diagram of a propagation tuned oscillator(PTO) incorporating the zero-crossing receiver 640 for operation incontinuous wave mode. In particular, with respect to FIG. 4, itillustrates closed loop measurement of the transit time 412 ofultrasound waves 414 within the waveguide 408 by the operation of thepropagation tuned oscillator 416. This example is for operation incontinuous wave mode. The system can also be operated in pulse mode anda pulse-echo mode. Pulse mode and pulsed echo-mode use a pulsed energywave. Pulse-echo mode uses reflection to direct an energy wave withinthe energy propagation medium. Briefly, the digital logic circuit 646digitizes the frequency of operation of the propagation tunedoscillator.

In continuous wave mode of operation a sensor comprising transducer 604,propagating structure 602, and transducer 606 is used to measure theparameter. In general, the parameter to be measured affects theproperties of the propagating medium. For example, an external force orcondition 612 is applied to propagating structure 602 that changes thelength of the waveguide in a path of a propagating energy wave. A changein length corresponds to a change in transit time 608 of the propagatingwave. Similarly, the length of propagating structure 602 corresponds tothe applied force 612. A length reduction corresponds to a higher forcebeing applied to the propagating structure 602. Conversely, a lengthincrease corresponds to a lowering of the applied force 612 to thepropagating structure 602. The length of propagating structure 602 ismeasured and is converted to force by way of a known length to forcerelationship.

Transducer 604 is an emitting device in continuous wave mode. The sensorfor measuring a parameter comprises transducer 604 coupled topropagating structure 602 at a first location. A transducer 606 iscoupled to propagating structure 602 at a second location. Transducer606 is a receiving transducer for capturing propagating energy waves. Inone embodiment, the captured propagated energy waves are electrical sinewaves 634 that are output by transducer 606.

A measurement sequence is initiated when control circuitry 618 closesswitch 620 coupling oscillator output 624 of oscillator 622 to the inputof amplifier 626. One or more pulses provided to amplifier 626 initiatesan action to propagate energy waves 610 having simple or complexwaveforms through energy propagating structure or medium 602. Amplifier626 comprises a digital driver 628 and matching network 630. In oneembodiment, amplifier 626 transforms the oscillator output of oscillator622 into sine waves of electrical waves 632 having the same repetitionrate as oscillator output 624 and sufficient amplitude to excitetransducer 604.

Emitting transducer 604 converts the sine waves 632 into energy waves610 of the same frequency and emits them at the first location intoenergy propagating structure or medium 602. The energy waves 610propagate through energy propagating structure or medium 602. Uponreaching transducer 606 at the second location, energy waves 610 arecaptured, sensed, or detected. The captured energy waves are convertedby transducer 606 into sine waves 634 that are electrical waves havingthe same frequency.

Amplifier 636 comprises a pre-amplifier 634 and zero-cross receiver 640.Amplifier 636 converts the sine waves 634 into digital pulses 642 ofsufficient duration to sustain the behavior of the closed loop circuit.Control circuitry 618 responds to digital pulses 642 from amplifier 636by opening switch 620 and closing switch 644. Opening switch 620decouples oscillator output 624 from the input of amplifier 626. Closingswitch 644 creates a closed loop circuit coupling the output ofamplifier 636 to the input of amplifier 626 and sustaining the emission,propagation, and detection of energy waves through energy propagatingstructure or medium 602.

An equilibrium state is attained by maintaining unity gain around thisclosed loop circuit wherein sine waves 632 input into transducer 604 andsine waves 634 output by transducer 606 are in phase with a small butconstant offset. Transducer 606 as disclosed above, outputs the sinewaves 634 upon detecting energy waves propagating to the secondlocation. In the equilibrium state, an integer number of energy waves610 propagate through energy propagating structure or medium 602.

Movement or changes in the physical properties of energy propagatingstructure or medium 602 change a transit time 608 of energy waves 610.The transit time 608 comprises the time for an energy wave to propagatefrom the first location to the second location of propagating structure602. Thus, the change in the physical property of propagating structure602 results in a corresponding time period change of the energy waves610 within energy propagating structure or medium 602. These changes inthe time period of the energy waves 610 alter the equilibrium point ofthe closed loop circuit and frequency of operation of the closed loopcircuit. The closed loop circuit adjusts such that sine waves 632 and634 correspond to the new equilibrium point. The frequency of energywaves 610 and changes to the frequency correlate to changes in thephysical attributes of energy propagating structure or medium 602.

The physical changes may be imposed on energy propagating structure 602by external forces or conditions 612 thus translating the levels andchanges of the parameter or parameters of interest into signals that maybe digitized for subsequent processing, storage, and display.Translation of the operating frequency into digital binary numbersfacilitates communication, additional processing, storage, and displayof information about the level and changes in physical parameters ofinterest. Similarly, the frequency of energy waves 610 during theoperation of the closed loop circuit, and changes in this frequency, maybe used to measure movement or changes in physical attributes of energypropagating structure or medium 602.

Prior to measurement of the frequency or operation of the propagationtuned oscillator, control logic 618 loads the loop count into digitalcounter 650 that is stored in count register 648. The first digitalpulses 642 initiates closed loop operation within the propagation tunedoscillator and signals control circuit 618 to start measurementoperations. At the start of closed loop operation, control logic 618enables digital counter 650 and digital timer 652. In one embodiment,digital counter 650 decrements its value on the rising edge of eachdigital pulse output by zero-crossing receiver 640. Digital timer 652increments its value on each rising edge of clock pulses 656. When thenumber of digital pulses 642 has decremented, the value within digitalcounter 650 to zero a stop signal is output from digital counter 650.The stop signal disables digital timer 652 and triggers control circuit618 to output a load command to data register 654. Data register 654loads a binary number from digital timer 652 that is equal to the periodof the energy waves or pulses times the value in counter 648 divided byclock period 656. With a constant clock period 656, the value in dataregister 654 is directly proportional to the aggregate period of theenergy waves or pulses accumulated during the measurement operation.Duration of the measurement operation and the resolution of measurementsmay be adjusted by increasing or decreasing the value preset in thecount register 648.

FIG. 7 is a sensor interface diagram incorporating the integratedzero-crossing receiver 200 in a pulse multiplexing arrangement formaintaining positive closed-loop feedback in accordance with oneembodiment. In one embodiment, the circuitry other than the sensor isintegrated on an application specific integrated circuit (ASIC). Thepositive closed-loop feedback is illustrated by the bold line path.Initially, mux 702 is enabled to couple one or more digital pulses 704to the transducer driver 706. Transducer driver 706 generates a pulsesequence 708 corresponding to digital pulses 704. Analog mux 710 isenabled to couple pulse sequence 708 to the transmitter transducer 712.Transducer 712 is coupled to a medium at a first location. Transducer712 responds to pulse sequence 708 and generates corresponding energypulses 714 that are emitted into the medium at the first location. Theenergy pulses 714 propagate through the medium. A receiver transducer716 is located at a second location on the medium. Receiver transducer716 captures the energy pulses 714 and generates a corresponding signalof electrical pulses 718. Transducer 716 is coupled to a mux 720. Mux720 is enabled to couple to zero-cross receiver 200. Electrical pulses718 from transducer 716 are coupled to zero-cross receiver 200.Zero-cross receiver 200 counts zero crossings of electrical pulses 718to determine changes in phase and frequency of the energy pulsesresponsive to an applied force, as previously explained. Zero-crossreceiver 200 outputs a pulse sequence 722 corresponding to the detectedsignal frequency. Pulse sequence 722 is coupled to mux 702. Mux 702 isdecoupled from coupling digital pulses 704 to driver 706 upon detectionof pulses 722. Conversely, mux 702 is enabled to couple pulses 722 todriver 706 upon detection of pulses 722 thereby creating a positiveclosed-loop feedback path. Thus, in pulse mode, zero-cross receiver 200is part of the closed-loop feedback path that continues emission ofenergy pulses into the medium at the first location and detection at thesecond location to measure a transit time and changes in transit time ofpulses through the medium.

FIG. 8 is an exemplary block diagram of a propagation tuned oscillator(PTO) incorporating the zero-crossing receiver 640 for operation inpulse mode. In particular, with respect to FIG. 4, it illustrates closedloop measurement of the transit time 412 of ultrasound waves 414 withinthe waveguide 408 by the operation of the propagation tuned oscillator416. This example is for operation in pulse mode. The system can also beoperated in continuous wave mode and a pulse-echo mode. Continuous wavemode uses a continuous wave signal. Pulse-echo mode uses reflection todirect an energy wave within the energy propagation medium. Briefly, thedigital logic circuit 646 digitizes the frequency of operation of thepropagation tuned oscillator.

In pulse mode of operation, a sensor comprising transducer 604,propagating structure 602, and transducer 606 is used to measure theparameter. In general, the parameter to be measured affects theproperties of the propagating medium. For example, an external force orcondition 612 is applied to propagating structure 602 that changes thelength of the waveguide in a path of a propagating energy wave. A changein length corresponds to a change in transit time 608 of the propagatingwave. The length of propagating structure 602 is measured and isconverted to force by way of a known length to force relationship. Onebenefit of pulse mode operation is the use of a high magnitude pulsedenergy wave. In one embodiment, the magnitude of the energy wave decaysas it propagates through the medium. The use of a high magnitude pulseis a power efficient method to produce a detectable signal if the energywave has to traverse a substantial distance or is subject to a reductionin magnitude as it propagated due to the medium.

A measurement sequence is initiated when control circuitry 618 closesswitch 620 coupling oscillator output 624 of oscillator 622 to the inputof amplifier 626. One or more pulses provided to amplifier 626 initiatesan action to propagate energy waves 610 having simple or complexwaveforms through energy propagating structure or medium 602. Amplifier626 comprises a digital driver 628 and matching network 630. In oneembodiment, amplifier 626 transforms the oscillator output of oscillator622 into analog pulses of electrical waves 832 having the samerepetition rate as oscillator output 624 and sufficient amplitude toexcite transducer 604.

Emitting transducer 604 converts the analog pulses 832 into energy waves610 of the same frequency and emits them at a first location into energypropagating structure or medium 602. The energy waves 610 propagatethrough energy propagating structure or medium 602. Upon reachingtransducer 606 at the second location, energy waves 610 are captured,sensed, or detected. The captured energy waves are converted bytransducer 606 into analog pulses 834 that are electrical waves havingthe same frequency.

Amplifier 636 comprises a pre-amplifier 638 and zero-cross receiver 640.Amplifier 636 converts the analog pulses 834 into digital pulses 642 ofsufficient duration to sustain the behavior of the closed loop circuit.Control circuitry 618 responds to digital pulses 642 from amplifier 636by opening switch 620 and closing switch 644. Opening switch 620decouples oscillator output 624 from the input of amplifier 626. Closingswitch 644 creates a closed loop circuit coupling the output ofamplifier 636 to the input of amplifier 626 and sustaining the emission,propagation, and detection of energy waves through energy propagatingstructure or medium 602.

An equilibrium state is attained by maintaining unity gain around thisclosed loop circuit wherein pulses 832 input into transducer 604 andpulses 834 output by transducer 606 are in phase with a small butconstant offset. Transducer 606 as disclosed above, outputs the pulses834 upon detecting energy waves propagating to the second location. Inthe equilibrium state, an integer number of energy waves 610 propagatethrough energy propagating structure or medium 602.

Movement or changes in the physical properties of energy propagatingstructure or medium 602 change a transit time 608 of energy waves 610.The transit time 608 comprises the time for an energy wave to propagatefrom the first location to the second location of propagating structure602. Thus, the change in the physical property of propagating structure602 results in a corresponding time period change of the energy waves610 within energy propagating structure or medium 602. These changes inthe time period of the energy waves 610 alter the equilibrium point ofthe closed loop circuit and frequency of operation of the closed loopcircuit. The closed loop circuit adjusts such that pulses 832 and 834correspond to the new equilibrium point. The frequency of energy waves610 and changes to the frequency correlate to changes in the physicalattributes of energy propagating structure or medium 602.

The physical changes may be imposed on energy propagating structure 602by external forces or conditions 612 thus translating the levels andchanges of the parameter or parameters of interest into signals that maybe digitized for subsequent processing, storage, and display.Translation of the operating frequency into digital binary numbersfacilitates communication, additional processing, storage, and displayof information about the level and changes in physical parameters ofinterest as disclosed in more detail hereinabove. Similarly, thefrequency of energy waves 610 during the operation of the closed loopcircuit, and changes in this frequency, may be used to measure movementor changes in physical attributes of energy propagating structure ormedium 602.

FIG. 9 illustrates a block diagram of an edge-detect receiver circuit900 in accordance with an exemplary embodiment. In a first embodiment,edge-detect receiver 900 is provided to detect wave fronts of pulses ofenergy waves. This enables capturing of parameters including, but notlimited to, transit time, phase, or frequency of the energy waves.Circuitry of the integrated edge-detect receiver 900 provides rapidon-set detection and quickly responds to the arrival of an energy pulse.It reliably triggers thereafter a digital output pulse at a same pointon the initial wave front of each captured energy pulse or pulsed energywave. The digital pulse can be optimally configured to output withminimal and constant delay. The edge-detect receiver 900 can isolate andprecisely detect the specified point on the initial energy wave or thewave front in the presence of interference and distortion signalsthereby overcoming problems commonly associated with detecting one ofmultiple generated complex signals in energy propagating mediums. Theedge-detect receiver 900 performs these functions accurately over a widerange of amplitudes including very low-level energy pulses.

In a second embodiment, the edge-detect receiver 900 is incorporatedwithin a propagation tuned oscillator (PTO) to maintain positiveclosed-loop feedback when operating in a pulse or pulse-echo mode. Theedge-detect receiver 900 can be integrated with other circuitry of thePTO by multiplexing input and output circuitry to achieve ultralow-power and small compact size. Integration of the circuitry of thePTO with the edge-detect receiver provides the benefit of increasingsensitivity to low-level signals.

The block diagram illustrates one embodiment of a low power edge-detectreceiver circuit 900 with superior performance at low signal levels. Theedge-detect receiver 900 comprises a preamplifier 912, a differentiator914, a digital pulse circuit 916 and a deblank circuit 918. Theedge-detect receiver circuit 900 can be implemented in discrete analogcomponents, digital components or combination thereof. In oneembodiment, edge-detect receiver 900 is integrated into an ASIC as partof a sensor system described hereinbelow. The edge-detect receivercircuit 900 practices measurement methods that detect energy pulses orpulsed energy waves at specified locations and under specifiedconditions to enable capturing parameters including, but not limited to,transit time, phase, frequency, or amplitude of energy pulses. A briefdescription of the method of operation is as follows. In a non-limitingexample, a pre-amplifier triggers a comparator circuit responsive tosmall changes in the slope of an input signal. The comparator and otheredge-detect circuitry responds rapidly with minimum delay. Detection ofsmall changes in the input signal assures rapid detection of the arrivalof a pulse of energy waves. The minimum phase design reduces extraneousdelay thereby introducing less variation into the measurement of thetransit time, phase, frequency, or amplitude of the incoming energypulses.

An input 920 of edge-detect receiver 900 is coupled to pre-amplifier912. As an example, the incoming wave 910 to the edge-detect receivercircuit 900 can be received from an electrical connection, antenna, ortransducer. The incoming wave 910 is amplified by pre-amplifier 912,which assures adequate sensitivity to small signals. Differentiatorcircuitry 914 monitors the output of pre-amplifier 912 and triggersdigital pulse circuitry 916 whenever a signal change corresponding to apulsed energy wave is detected. For example, a signal change thatidentifies the pulsed energy wave is the initial wave front or theleading edge of the pulsed energy wave. In one arrangement,differentiator 914 detects current flow, and more specifically changesin the slope of the energy wave 910 by detecting small changes incurrent flow instead of measuring changes in voltage level to achieverapid detection of slope. Alternatively, differentiator 914 can beimplemented to trigger on changes in voltage. Together, preamplifier 912and differentiator 916 monitor the quiescent input currents for thearrival of wave front of energy wave(s) 910. Preamplifier 912 anddifferentiator 916 detect the arrival of low level pulses of energywaves as well as larger pulses of energy waves. This detectionmethodology achieves superior performance for very low level signals.Differentiator circuitry 912 triggers digital pulse circuitry 916whenever current flow driven by the initial signal ramp of the incomingwave 910 is detected. The digital pulse is coupled to deblank circuit918 that desensitizes pre-amplifier 912. For example, thedesensitization of pre-amplifier 912 can comprise a reduction in gain,decoupling of input 920 from energy wave 910, or changing the frequencyresponse. The deblank circuit 918 also disregards voltage or currentlevels for a specified or predetermined duration of time to effectivelyskip over the interference sections or distorted portions of the energywave 910. In general, energy wave 910 can comprise more than one changein slope and is typically a damped wave form. Additional signals orwaves of the pulsed energy wave on the input 920 of pre-amplifier 912are not processed during the preset blanking period. In this example,the digital output pulse 928 can then be coupled to signal processingcircuitry as explained hereinbelow. In one embodiment, the electroniccomponents are operatively coupled as blocks within an integratedcircuit. As will be shown ahead, this integration arrangement performsits specific functions efficiently with a minimum number of components.This is because the circuit components are partitioned betweenstructures within an integrated circuit and discrete components, as wellas innovative partitioning of analog and digital functions, to achievethe required performance with a minimum number of components and minimumpower consumption.

FIG. 10 illustrates a block diagram of the edge-detect receiver circuit900 coupled to a sensing assembly 1000. The pre-amplifier 912 and thedigital pulse circuit 916 are shown for reference and discussion. Thesensing assembly 1000 comprises a transmitter transducer 1002, an energypropagating medium 1004, and a receiver transducer 1006. The transmittertransducer 1002 is coupled to propagating medium 1004 at a firstlocation. The receiver transducer 1006 is coupled to energy propagatingmedium 1004 at a second location. Alternatively, a reflecting surfacecan replace receiver transducer 1006. The reflecting surface reflects anenergy wave back towards the first location. Transducer 1006 can beenabled to be a transmitting transducer and a receiving transducerthereby saving the cost of a transducer. As will be explained ahead infurther detail, the sensing assembly 1000 in one embodiment is part of asensory device that assess loading, in particular, the externallyapplied forces 1008 on the sensing assembly 1000. A transducer drivercircuit (not shown) drives the transmitter transducer 1002 of thesensing assembly 1000 to produce energy waves 1010 that are directedinto the energy propagating medium 1004. In the non-limiting example,changes in the energy propagating medium 1004 due to the externallyapplied forces 1008 change the frequency, phase, and transit time 1012of energy waves 1010 propagating from the first location to the secondlocation of energy propagating medium 1004. The integrated edge-detectreceiver circuit 900 is coupled to the receiver transducer 1006 todetect edges of the reproduced energy wave 910 and trigger the digitalpulse 928. In general, the timing of the digital pulse 928 conveys theparameters of interest (e.g., distance, force weight, strain, pressure,wear, vibration, viscosity, density, direction, displacement, etc.)related to the change in energy propagating structure 1004 due to anexternal parameter. For example, sensing assembly 1000 placed in a kneejoint as described hereinabove.

Measurement methods that rely on the propagation of energy pulsesrequire the detection of energy pulses at specified locations or underspecified conditions to enable capturing parameters including, but notlimited to, transit time, phase, frequency, or amplitude of the energypulses. Measurement methods that rely on such propagation of energywaves 1010 or pulses of energy waves are required to achieve highlyaccurate and controlled detection of energy waves or pulses. Moreover,pulses of energy waves may contain multiple energy waves with complexwaveforms therein leading to potential ambiguity of detection. Inparticular, directing energy waves 1010 into the energy propagatingstructure 1004 can generate interference patterns caused by nulls andresonances of the waveguide, as well as characteristics of the generatedenergy wave 1010. These interference patterns can generate multiplyexcited waveforms that result in distortion of the edges of the originalenergy wave. To reliably detect the arrival of a pulse of energy waves,the edge-detect receiver 900 only responds to the leading edge of thefirst energy wave within each pulse. This is achieved in part byblanking the edge-detect circuitry 900 for the duration of each energypulse. As an example, the deblank circuit 918 disregards voltage orcurrent levels for a specified duration of time to effectively skip overthe interference sections or distorted portions of the waveform.

FIG. 11 is a sensor interface diagram incorporating the edge-detectreceiver circuit 900 in a pulse-echo multiplexing arrangement formaintaining positive closed-loop feedback in accordance with oneembodiment. The positive closed-loop feedback is illustrated by the boldline path. Initially, multiplexer (mux) 1102 receives as input a digitalpulse 1104, which is passed to the transducer driver 1106 to produce thepulse sequence 1108. Analog multiplexer (mux) 1110 receives pulsesequence 1108, which is passed to the transducer 1112 to generate energypulses 1114. Energy pulses 1114 are emitted into a first location of amedium and propagate through the medium. In the pulse-echo example,energy pulses 1114 are reflected off a surface 1116 at a second locationof the medium, for example, the end of a waveguide or reflector, andechoed back to the transducer 1112. The transducer 1112 proceeds to thencapture the reflected pulse echo. In pulsed echo mode, the transducer1112 performs as both a transmitter and a receiver. As disclosed above,transducer 1112 toggles back and forth between emitting and receivingenergy waves. Transducer 1112 captures the reflected echo pulses, whichare coupled to analog mux 1110 and directed to the edge-detect receiver900. The captured reflected echo pulses is indicated by electrical waves1120. Edge-detect receiver 900 locks on pulse edges corresponding to thewave front of a propagated energy wave to determine changes in phase andfrequency of the energy pulses 1114 responsive to an applied force, aspreviously explained. Among other parameters, it generates a pulsesequence 1118 corresponding to the detected signal frequency. The pulsesequence 1118 is coupled to mux 1102 and directed to driver 1106 toinitiate one or more energy waves being emitted into the medium bytransducer 1112. Pulse 1104 is decoupled from being provided to driver1106. Thus, a positive closed loop feedback is formed that repeatablyemits energy waves into the medium until mux 1102 prevents a signal frombeing provided to driver 1106. The edge-detect receiver 900 is coupledto a second location of the medium and is in the feedback path. Theedge-detect receiver 900 initiates a pulsed energy wave being providedat the first location of the medium upon detecting a wave front at thesecond location when the feedback path is closed.

FIG. 12 is an exemplary block diagram of a propagation tuned oscillator(PTO) incorporating the edge-detect receiver circuit 900 for operationin pulse echo mode. In particular, with respect to FIG. 4, itillustrates closed loop measurement of the transit time 412 ofultrasound waves 414 within the waveguide 408 by the operation of thepropagation tuned oscillator 416. This example is for operation in apulse echo mode. The system can also be operated in pulse mode and acontinuous wave mode. Pulse mode does not use a reflected signal.Continuous wave mode uses a continuous signal. Briefly, the digitallogic circuit 1246 digitizes the frequency of operation of thepropagation tuned oscillator.

In pulse-echo mode of operation a sensor comprising transducer 1204,propagating structure 1202, and reflecting surface 1206 is used tomeasure the parameter. In general, the parameter to be measured affectsthe properties of the propagating medium. For example, an external forceor condition 1212 is applied to propagating structure 1202 that changesthe length of the waveguide in a path of a propagating energy wave. Achange in length corresponds to a change in transit time of thepropagating wave. Similarly, the length of propagating structure 1202corresponds to the applied force 1212. A length reduction corresponds toa higher force being applied to the propagating structure 1202.Conversely, a length increase corresponds to a lowering of the appliedforce 1212 to the propagating structure 1202. The length of propagatingstructure 1202 is measured and is converted to force by way of a knownlength to force relationship.

Transducer 1204 is both an emitting device and a receiving device inpulse-echo mode. The sensor for measuring a parameter comprisestransducer 1204 coupled to propagating structure 1202 at a firstlocation. A reflecting surface is coupled to propagating structure 1202at a second location. Transducer 1204 has two modes of operationcomprising an emitting mode and receiving mode. Transducer 1204 emits anenergy wave into the propagating structure 1202 at the first location inthe emitting mode. The energy wave propagates to a second location andis reflected by reflecting surface 1206. The reflected energy wave isreflected towards the first location and transducer 1204 subsequentlygenerates a signal in the receiving mode corresponding to the reflectedenergy wave.

A measurement sequence in pulse echo mode is initiated when controlcircuitry 1218 closes switch 1220 coupling digital output 1224 ofoscillator 1222 to the input of amplifier 1226. One or more pulsesprovided to amplifier 1226 starts a process to emit one or more energywaves 1210 having simple or complex waveforms into energy propagatingstructure or medium 1202. Amplifier 1226 comprises a digital driver 1228and matching network 1230. In one embodiment, amplifier 1226 transformsthe digital output of oscillator 1222 into pulses of electrical waves1232 having the same repetition rate as digital output 1224 andsufficient amplitude to excite transducer 1204.

Transducer 1204 converts the pulses of electrical waves 1232 into pulsesof energy waves 1210 of the same repetition rate and emits them intoenergy propagating structure or medium 1202. The pulses of energy waves1210 propagate through energy propagating structure or medium 1202 asshown by arrow 1214 towards reflecting surface 1206. Upon reachingreflecting surface 1206, energy waves 1210 are reflected by reflectingsurface 1206. Reflected energy waves propagate towards transducer 1204as shown by arrow 1216. The reflected energy waves are detected bytransducer 1204 and converted into pulses of electrical waves 1234having the same repetition rate.

Amplifier 1236 comprises a pre-amplifier 1234 and edge-detect receiver1240. Amplifier 1236 converts the pulses of electrical waves 1234 intodigital pulses 1242 of sufficient duration to sustain the pulse behaviorof the closed loop circuit. Control circuitry 1218 responds to digitaloutput pulses 1242 from amplifier 1236 by opening switch 1220 andclosing switch 1244. Opening switch 1220 decouples oscillator output1224 from the input of amplifier 1226. Closing switch 1244 creates aclosed loop circuit coupling the output of amplifier 1236 to the inputof amplifier 1226 and sustaining the emission, propagation, anddetection of energy pulses through energy propagating structure ormedium 1202.

An equilibrium state is attained by maintaining unity gain around thisclosed loop circuit wherein electrical waves 1232 input into transducer1204 and electrical waves 1234 output by transducer 1204 are in phasewith a small but constant offset. Transducer 1204 as disclosed above,outputs the electrical waves 1234 upon detecting reflected energy wavesreflected from reflecting surface 1206. In the equilibrium state, aninteger number of pulses of energy waves 1210 propagate through energypropagating structure or medium 1202.

Movement or changes in the physical properties of energy propagatingstructure or medium 1202 change a transit time 1208 of energy waves1210. The transit time 1208 comprises the time for an energy wave topropagate from the first location to the second location of propagatingstructure 1202 and the time for the reflected energy wave to propagatefrom the second location to the first location of propagating structure1202. Thus, the change in the physical property of propagating structure1202 results in a corresponding time period change of the energy waves1210 within energy propagating structure or medium 1202. These changesin the time period of the repetition rate of the energy pulses 1210alter the equilibrium point of the closed loop circuit and repetitionrate of operation of the closed loop circuit. The closed loop circuitadjusts such that electrical waves 1232 and 1234 correspond to the newequilibrium point. The repetition rate of energy waves 1210 and changesto the repetition rate correlate to changes in the physical attributesof energy propagating structure or medium 1202.

The physical changes may be imposed on energy propagating structure 1202by external forces or conditions 1212 thus translating the levels andchanges of the parameter or parameters of interest into signals that maybe digitized for subsequent processing, storage, and display.Translation of the operating frequency into digital binary numbersfacilitates communication, additional processing, storage, and displayof information about the level and changes in physical parameters ofinterest. Similarly, the frequency of energy waves 1210 during theoperation of the closed loop circuit, and changes in this frequency, maybe used to measure movement or changes in physical attributes of energypropagating structure or medium 1202.

Prior to measurement of the frequency or operation of the propagationtuned oscillator, control logic 1218 loads the loop count into digitalcounter 1250 that is stored in count register 1248. The first digitalpulses 1242 initiates closed loop operation within the propagation tunedoscillator and signals control circuit 1218 to start measurementoperations. At the start of closed loop operation, control logic 1218enables digital counter 1250 and digital timer 1252. In one embodiment,digital counter 1250 decrements its value on the rising edge of eachdigital pulse output by edge-detect receiver 1240. Digital timer 1252increments its value on each rising edge of clock pulses 1256. When thenumber of digital pulses 1242 has decremented, the value within digitalcounter 1250 to zero a stop signal is output from digital counter 1250.The stop signal disables digital timer 1252 and triggers control circuit1218 to output a load command to data register 1254. Data register 1254loads a binary number from digital timer 1252 that is equal to theperiod of the energy waves or pulses times the value in counter 1248divided by clock period 1256. With a constant clock period 1256, thevalue in data register 1254 is directly proportional to the aggregateperiod of the energy waves or pulses accumulated during the measurementoperation. Duration of the measurement operation and the resolution ofmeasurements may be adjusted by increasing or decreasing the valuepreset in the count register 1248.

FIG. 13 is a simplified cross-sectional view of a sensing module 1301 inaccordance with an exemplary embodiment. The sensing module (orassemblage) is an electro-mechanical assembly comprising electricalcomponents and mechanical components that when configured and operatedin accordance with a sensing mode performs as a positive feedbackclosed-loop measurement system. The measurement system can preciselymeasure applied forces, such as loading, on the electro-mechanicalassembly. The sensing mode can be a continuous mode, a pulse mode, or apulse echo-mode.

In one embodiment, the electrical components can include ultrasoundresonators or transducers 405 and 406, ultrasound waveguides 403, andsignal processing electronics 1310, but are not limited to these. Themechanical components can include biasing springs 1332, spring retainersand posts, and load platforms 1306, but are not limited to these. Theelectrical components and mechanical components can be inter-assembled(or integrated) onto a printed circuit board 1336 to operate as acoherent ultrasonic measurement system within sensing module 1301 andaccording to the sensing mode. As will be explained ahead in moredetail, the signal processing electronics incorporate a propagationtuned oscillator (PTO) or a phase locked loop (PLL) to control theoperating frequency of the ultrasound resonators or transducers forproviding high precision sensing. Furthermore, the signal processingelectronics incorporate detect circuitry that consistently detects anenergy wave after it has propagated through a medium. The detectioninitiates the generation of a new energy wave by an ultrasound resonatoror transducer that is coupled to the medium for propagationtherethrough. A change in transit time of an energy wave through themedium is measured and correlates to a change in material property ofthe medium due to one or more parameters applied thereto.

Sensing module 1301 comprises one or more assemblages 401 each comprisedone or more ultrasound resonators 405 and 406. As illustrated, waveguide403 is coupled between transducers (405, 406) and affixed to loadbearing or contacting surfaces 408. In one exemplary embodiment, anultrasound signal is coupled for propagation through waveguide 403. Thesensing module 1301 is placed, attached to, or affixed to, or within abody, instrument, or other physical system 1318 having a member ormembers 1316 in contact with the load bearing or contacting surfaces 408of the sensing module 401. This arrangement facilitates translating theparameters of interest into changes in the length or compression orextension of the waveguide or waveguides 403 within the sensing module1301 and converting these changes in length into electrical signals.This facilitates capturing data, measuring parameters of interest anddigitizing that data, and then subsequently communicating that datathrough antenna 1334 to external equipment with minimal disturbance tothe operation of the body, instrument, appliance, vehicle, equipment, orphysical system 1318 for a wide range of applications.

The sensing module 401 supports three modes of operation of energy wavepropagation and measurement: reflectance, unidirectional, andbi-directional. These modes can be used as appropriate for eachindividual application. In unidirectional and bi-directional modes, achosen ultrasound resonator or transducer is controlled to emit pulsesof ultrasound waves into the ultrasound waveguide and one or more otherultrasound resonators or transducers are controlled to detect thepropagation of the pulses of ultrasound waves at a specified location orlocations within the ultrasound waveguide. In reflectance or pulse-echomode, a single ultrasound or transducer emits pulses of ultrasound wavesinto waveguide 403 and subsequently detects pulses of echo waves afterreflection from a selected feature or termination of the waveguide. Inpulse-echo mode, echoes of the pulses can be detected by controlling theactions of the emitting ultrasound resonator or transducer to alternatebetween emitting and detecting modes of operation. Pulse and pulse-echomodes of operation may require operation with more than one pulsedenergy wave propagating within the waveguide at equilibrium.

Many parameters of interest within physical systems or bodies can bemeasured by evaluating changes in the transit time of energy pulses. Thefrequency, as defined by the reciprocal of the average period of acontinuous or discontinuous signal, and type of the energy pulse isdetermined by factors such as distance of measurement, medium in whichthe signal travels, accuracy required by the measurement, precisionrequired by the measurement, form factor of that will function with thesystem, power constraints, and cost. The physical parameter orparameters of interest can include, but are not limited to, measurementof load, force, pressure, displacement, density, viscosity, localizedtemperature. These parameters can be evaluated by measuring changes inthe propagation time of energy pulses or waves relative to orientation,alignment, direction, or position as well as movement, rotation, oracceleration along an axis or combination of axes by wireless sensingmodules or devices positioned on or within a body, instrument,appliance, vehicle, equipment, or other physical system.

In the non-limiting example, pulses of ultrasound energy provideaccurate markers for measuring transit time of the pulses withinwaveguide 403. In general, an ultrasonic signal is an acoustic signalhaving a frequency above the human hearing range (e.g. >20 KHz)including frequencies well into the megahertz range. In one embodiment,a change in transit time of an ultrasonic energy pulse corresponds to adifference in the physical dimension of the waveguide from a previousstate. For example, a force or pressure applied across the knee jointcompresses waveguide 403 to a new length and changes the transit time ofthe energy pulse When integrated as a sensing module and inserted orcoupled to a physical system or body, these changes are directlycorrelated to the physical changes on the system or body and can bereadily measured as a pressure or a force.

FIG. 14 is an exemplary assemblage 1400 for illustrating reflectance andunidirectional modes of operation in accordance with an exemplaryembodiment. It comprises one or more transducers 1402, 1404, and 1406,one or more waveguides 1414, and one or more optional reflectingsurfaces 1416. The assemblage 1400 illustrates propagation of ultrasoundwaves 1418 within the waveguide 1414 in the reflectance andunidirectional modes of operation. Either ultrasound resonator ortransducer 1402 and 1404 in combination with interfacing material ormaterials 1408 and 1410, if required, can be selected to emit ultrasoundwaves 1418 into the waveguide 1414.

In unidirectional mode, either of the ultrasound resonators ortransducers for example 1402 can be enabled to emit ultrasound waves1418 into the waveguide 1414. The non-emitting ultrasound resonator ortransducer 1404 is enabled to detect the ultrasound waves 1418 emittedby the ultrasound resonator or transducer 1402.

In reflectance mode, the ultrasound waves 1418 are detected by theemitting ultrasound resonator or transducer 1402 after reflecting from asurface, interface, or body at the opposite end of the waveguide 1414.In this mode, either of the ultrasound resonators or transducers 1402 or1404 can be selected to emit and detect ultrasound waves. Additionalreflection features 1416 can be added within the waveguide structure toreflect ultrasound waves. This can support operation in a combination ofunidirectional and reflectance modes. In this mode of operation, one ofthe ultrasound resonators, for example resonator 1402 is controlled toemit ultrasound waves 1418 into the waveguide 1414. Another ultrasoundresonator or transducer 1406 is controlled to detect the ultrasoundwaves 1418 emitted by the emitting ultrasound resonator 1402 (ortransducer) subsequent to their reflection by reflecting feature 1416.

FIG. 15 is an exemplary assemblage 1500 that illustrates propagation ofultrasound waves 1510 within the waveguide 1506 in the bi-directionalmode of operation of this assemblage. In this mode, the selection of theroles of the two individual ultrasound resonators (1502, 1504) ortransducers affixed to interfacing material 1520 and 1522, if required,are periodically reversed. In the bi-directional mode the transit timeof ultrasound waves propagating in either direction within the waveguide1506 can be measured. This can enable adjustment for Doppler effects inapplications where the sensing module 1508 is operating while in motion1516. Furthermore, this mode of operation helps assure accuratemeasurement of the applied load, force, pressure, or displacement bycapturing data for computing adjustments to offset this external motion1516. At least one embodiment includes situations wherein the body,instrument, appliance, vehicle, equipment, or other physical system1514, is itself operating or moving during sensing of load, pressure, ordisplacement. Similarly, the capability can also correct in situationwhere the body, instrument, appliance, vehicle, equipment, or otherphysical system, is causing the portion 1512 of the body, instrument,appliance, vehicle, equipment, or other physical system being measuredto be in motion 1516 during sensing of load, force, pressure, ordisplacement. Other adjustments to the measurement for physical changesto system 1514 are contemplated and can be compensated for in a similarfashion. For example, temperature of system 1514 can be measured and alookup table or equation having a relationship of temperature versustransit time can be used to normalize measurements. Differentialmeasurement techniques can also be used to cancel many types of commonfactors as is known in the art.

The use of waveguide 1506 enables the construction of low cost sensingmodules and devices over a wide range of sizes, including highly compactsensing modules, disposable modules for bio-medical applications, anddevices, using standard components and manufacturing processes. Theflexibility to construct sensing modules and devices with very highlevels of measurement accuracy, repeatability, and resolution that canscale over a wide range of sizes enables sensing modules and devices tothe tailored to fit and collect data on the physical parameter orparameters of interest for a wide range of medical and non-medicalapplications.

For example, sensing modules or devices may be placed on or within, orattached or affixed to or within, a wide range of physical systemsincluding, but not limited to instruments, appliances, vehicles,equipments, or other physical systems as well as animal and humanbodies, for sensing the parameter or parameters of interest in real timewithout disturbing the operation of the body, instrument, appliance,vehicle, equipment, or physical system.

In addition to non-medical applications, examples of a wide range ofpotential medical applications may include, but are not limited to,implantable devices, modules within implantable devices, modules ordevices within intra-operative implants or trial inserts, modules withininserted or ingested devices, modules within wearable devices, moduleswithin handheld devices, modules within instruments, appliances,equipment, or accessories of all of these, or disposables withinimplants, trial inserts, inserted or ingested devices, wearable devices,handheld devices, instruments, appliances, equipment, or accessories tothese devices, instruments, appliances, or equipment. Many physiologicalparameters within animal or human bodies may be measured including, butnot limited to, loading within individual joints, bone density,movement, various parameters of interstitial fluids including, but notlimited to, viscosity, pressure, and localized temperature withapplications throughout the vascular, lymph, respiratory, and digestivesystems, as well as within or affecting muscles, bones, joints, and softtissue areas. For example, orthopedic applications may include, but arenot limited to, load bearing prosthetic components, or provisional ortrial prosthetic components for, but not limited to, surgical proceduresfor knees, hips, shoulders, elbows, wrists, ankles, and spines; anyother orthopedic or musculoskeletal implant, or any combination ofthese.

FIG. 16 is an exemplary cross-sectional view of a sensor element 1600 toillustrate changes in the propagation of ultrasound waves 1614 withchanges in the length of a waveguide 1606. In general, the measurementof a parameter is achieved by relating displacement to the parameter. Inone embodiment, the displacement required over the entire measurementrange is measured in microns. For example, an external force 1608compresses waveguide 1606 thereby changing the length of waveguide 1606.Sensing circuitry (not shown) measures propagation characteristics ofultrasonic signals in the waveguide 1606 to determine the change in thelength of the waveguide 1606. These changes in length change in directproportion to the parameters of interest thus enabling the conversion ofchanges in the parameter or parameters of interest into electricalsignals.

As illustrated, external force 1608 compresses waveguide 1606 and movesthe transducers 1602 and 1604 closer to one another by a distance 1610.This changes the length of waveguide 1606 by distance 1612 of thewaveguide propagation path between transducers 1602 and 1604. Dependingon the operating mode, the sensing circuitry measures the change inlength of the waveguide 1606 by analyzing characteristics of thepropagation of ultrasound waves within the waveguide.

One interpretation of FIG. 16 illustrates waves emitting from transducer1602 at one end of waveguide 1606 and propagating to transducer 1604 atthe other end of the waveguide 1606. The interpretation includes theeffect of movement of waveguide 1606 and thus the velocity of wavespropagating within waveguide 1606 (without changing shape or width ofindividual waves) and therefore the transit time between transducers1602 and 1604 at each end of the waveguide. The interpretation furtherincludes the opposite effect on waves propagating in the oppositedirection and is evaluated to estimate the velocity of the waveguide andremove it by averaging the transit time of waves propagating in bothdirections.

Changes in the parameter or parameters of interest are measured bymeasuring changes in the transit time of energy pulses or waves withinthe propagating medium. Closed loop measurement of changes in theparameter or parameters of interest is achieved by modulating therepetition rate of energy pulses or the frequency of energy waves as afunction of the propagation characteristics of the elastic energypropagating structure.

In a continuous wave mode of operation, a phase detector (not shown)evaluates the frequency and changes in the frequency of resonantultrasonic waves in the waveguide 1606. As will be described below,positive feedback closed-loop circuit operation in continuous wave (CW)mode adjusts the frequency of ultrasonic waves 1614 in the waveguide1606 to maintain a same number or integer number of periods ofultrasonic waves in the waveguide 1606. The CW operation persists aslong as the rate of change of the length of the waveguide is not sorapid that changes of more than a quarter wavelength occur before thefrequency of the Propagation Tuned Oscillator (PTO) can respond. Thisrestriction exemplifies the difference between the performance of a PTOand a Phase Locked Loop (PLL). Assuming the transducers are producingultrasonic waves, for example, at 2.4 MHz, the wavelength in air,assuming a velocity of 343 microns per microsecond, is about 143μ,although the wavelength within a waveguide may be longer than inunrestricted air.

In a pulse mode of operation, the phase detector measures a time offlight (TOF) between when an ultrasonic pulse is transmitted bytransducer 1602 and received at transducer 1604. The time of flightdetermines the length of the waveguide propagating path, and accordinglyreveals the change in length of the waveguide 1606. In anotherarrangement, differential time of flight measurements (or phasedifferences) can be used to determine the change in length of thewaveguide 1606. A pulse consists of a pulse of one or more waves. Thewaves may have equal amplitude and frequency (square wave pulse) or theymay have different amplitudes, for example, decaying amplitude(trapezoidal pulse) or some other complex waveform. The PTO is holdingthe phase of the leading edge of the pulses propagating through thewaveguide constant. In pulse mode operation the PTO detects the leadingedge of the first wave of each pulse with an edge-detect receiver ratherthan a zero-crossing receiver circuitry as used in CW mode.

FIG. 18 illustrates a block diagram of a medical sensing system 1800 inaccordance with an example embodiment. The medical sensing systemoperates similar to the systems described in FIG. 4, FIG. 6, FIG. 8, andFIG. 12 to measure a medical parameter. The sensor of system 1800 iscapacitor 1802. Capacitor 1802 is a variable capacitor that varies withthe medical parameter being measured. A capacitance value of capacitor1802 correlates to a value of the parameter. In a first example, theparameter being measured is temperature. The capacitance of capacitor1802 is coupled to the temperature to be measured. The capacitance ofcapacitor 1802 at “temperature” can be accurately measured by system1800 and correlated back to a temperature value. Another example of aparameter is a force, pressure, or load. In one embodiment, the force,pressure, or load can be applied to capacitor 1802. The capacitance ofcapacitor 1802 at the “force, pressure, or load” is measured by system1800 and correlated back to a force, pressure, or load value. In eitherexample, the capacitance will change by a known manner over theparameter measurement range. In general, the change in capacitance overthe parameter measurement range occurs in a regular manner.Irregularities in capacitance change within the parameter System 1800can be calibrated over the parameter measurement range to account forany irregularities in capacitance change or to further refinemeasurement accuracy.

System 1800 comprises a capacitor 1802, a signal generator 1804, adigital clock 1806, a digital counter 1808, a digital timer 1810, acounter register 1812, and a data register 1814. Signal generator 1804is coupled to capacitor 1802 and has an output for providing a signal.Signal generator 1804 generates a signal 1816 or waveform thatcorresponds to the capacitance of capacitor 1802. The signal 1816changes as the capacitance of capacitor 1802 changes. For example, atime period of a measurement cycle of signal 1816 can relate to thecapacitance of capacitor 1802.

In one embodiment, signal generator 1804 is an oscillator. A digitalclock 1806 is coupled to digital counter 1808 and digital timer 1810.Digital clock 1806 provides a clock signal to digital counter 1808 anddigital timer 1810 during a measurement sequence. Digital counter 1808couples to counter register 1812 and couples to the output of signalgenerator 1804. Counter register 1812 provides a predetermined countcorresponding to the measurement sequence. In general, measurementaccuracy can be increased by raising the predetermined count. Digitalcounter 1808 receives the predetermined count from counter register1812. After initiating the measurement sequence the digital countercompares the number of measurement cycles at the output of signalgenerator 1804 to the predetermined count. The measurement sequence endswhen the count of measurement cycles equals the predetermined count. Inone embodiment, each measurement cycle output by signal generator 1804decrements digital counter 1808 until a zero count is reached whichsignifies an end of the measurement sequence. Digital timer 1810measures a time period of the measurement sequence. In other words,digital timer 1810 measures an elapsed time required for signalgenerator 1804 to output the predetermined count of measurement cycles.Data register 1814 couples to digital timer 1810 and stores a valuecorresponding to the time period or elapsed time of the measurementsequence. The elapsed time of the measurement sequence corresponds to astatistically large number of measurements of capacitor 1802. Theelapsed time corresponds to an aggregate of the predetermined count ofmeasurement cycles or capacitance measurements. The value stored in dataregister 1814 can be a translation of the elapsed time to a force,pressure, or load value. The parameter being measured should produce astable capacitance value during the time period of the measurementsequence.

FIG. 19 illustrates an oscillator 1900 generating a signal correspondingto a capacitor 1802 in accordance with an example embodiment. Oscillator1900 corresponds to signal generator 1804 of FIG. 18. Oscillator 1900 isan example of a circuit used to generate signal 1816 of FIG. 18.Oscillator 1900 comprises a current source 1902, a current source 1904,a comparator 1906, a switch 1908, a switch 1910, and a switch control1912. Capacitor 1802 is coupled to current sources 1902 and 1904.Current sources 1902 and 1904 respectively source and sink current fromcapacitor 1802. Current source 1902 sources a current I. Current source1904 sinks a current 21 or twice the current provided by current source1902. Switch 1910 enables current source 1904 to sink current whencoupled to ground. Comparator 1906 includes a positive input coupled tocapacitor 1802, a negative input coupled to switch 1908, and an output.The output of comparator 1906 couples to switch control 1912. Switchcontrol 1912 couples to switches 1908 and 1910 to control switchposition. The output of comparator 1906 is a control signal to switchcontrol 1912.

In general, current sources 1902 and 1904 respectively charge anddischarge capacitor 1802. Capacitor 1802 is charged by current source1902 when the output of comparator 1906 is in a low state. Switchcontrol 1912 opens switch 1910 and a reference voltage Vref is coupledto the negative input of comparator 1906 by switch 1908 when the outputof comparator 1906 transitions to the low state. The voltage oncapacitor 1802 rises as the current I from current source 1902 chargesthe capacitance. The slew rate of the change in voltage on the capacitoris related to the capacitance of capacitor 1802 and the current I. Theoutput of comparator 1906 transitions from a low state to a high statewhen the voltage on capacitor 1802 is greater than or equal to thereference voltage Vref. Switch control 1912 closes switch 1910 and areference voltage Vref/2 is coupled to the negative input of comparator1906 by switch 1908 when the output of comparator 1906 transitions tothe high state. The sink current of current source 1904 is 21 or twiceas large as the current sourced by current source 1902. Current source1904 sinks a current I from capacitor 1802 and an equal current fromcurrent source 1902. The voltage on capacitor 1802 falls as charge isremoved. The output of comparator changes from the high state to a lowstate when the voltage on the capacitor is less than or equal to thereference voltage Vref/2. In the example, voltage on capacitor 1802 willtransition between the reference voltages Vref and Vref/2. The slew rateof the rising edge and falling edge of the capacitor voltage issymmetrical. A repeating saw tooth pattern is generated by oscillator1900 until the sequence is stopped. A measurement cycle corresponds tothe time to generate a single triangle shaped waveform. The triangleshaped waveform constitutes the time to transition the voltage oncapacitor 1802 from Vref/2 to Vref and from Vref to Vref/2. It should benoted that the measurement cycle relates to the capacitance of capacitor1802. Increasing the capacitance of capacitor 1802 correspondinglyincreases the measurement cycle. Conversely, decreasing the capacitanceof capacitor 1802 correspondingly decreases the measurement cycle. Thesignal at the output of the comparator 1906 also corresponds to signal1816. Thus, a relation is established by the signal output by oscillator1900 to the capacitance of capacitor 1802.

Referring briefly to FIG. 1, a sensor 100 is coupled to themuscular-skeletal system. In the example, a prosthetic knee joint isillustrated and the sensor 100 is coupled to the knee region. Sensor 100can be capacitor 1802 coupled to the muscular-skeletal system. Capacitor1802 can be coupled to an articular surface of the prosthetic knee jointto measure a force, pressure, or load. In one embodiment, the force,pressure, or load applied to the articular surface is coupled tocapacitor 1802 whereby the capacitance varies with the force, pressure,or load applied thereto. Although a knee joint is shown, capacitor 1802and system 1800 of FIG. 18 can be used in medical devices, tools,equipment, and prosthetic components to measure parameters that affectcapacitance of capacitor 1802. Similarly, although a knee joint isdescribed as an example, capacitor 1802 can be integrated intomuscular-skeletal medical devices, tools, equipment, and prostheticcomponents to measure an applied force, pressure, or load. Moreover,capacitor 1802 and system 1800 of FIG. 18 is not limited to the knee butcan be integrated into prosthetic components for parameter measurementsuch as bone, tissue, shoulder, ankle, hip, knee, spine, elbow, hand,and foot.

Referring back to FIGS. 18 and 19, signal generator 1804 outputs arepeating waveform that corresponds to the capacitance of capacitor1802. Oscillator 1900 is an implementation of signal generator 1804 thatoscillates or generates a repeating waveform. In the example, oscillator1900 outputs a repeating sawtooth waveform that has symmetrical risingand falling edges. The measurement cycle of the waveform is the timerequired to transition from Vref/2 to Vref and transition back toVref/2. The time of the measurement cycle corresponds to the capacitanceof the capacitor. The time of each measurement cycle will besubstantially equal if the capacitance of capacitor 1802 remainsconstant during the measurement sequence. In one embodiment, counterregister 1812 is loaded with a predetermined count. The measurementsequence can be initiated at a predetermined point of the waveform. Forexample, a voltage Vref/2 can be detected to start on the waveform tostart the measurement sequence. Each subsequent time the voltage Vref/2is detected the digital counter 1808 is decremented. The measurementsequence ends when digital counter decrements to zero. Digital timer1810 measures the elapsed time of the measurement sequence correspondingto the predetermined count of measurement cycles of the sawtoothwaveform. Alternatively, the output of comparator 1906 can be used asthe oscillating or repeating waveform. A rising or falling edge of theoutput of comparator 1906 can be used to initiate and decrement digitalcounter 1808. The measurement sequence is configured to be initiatedduring a period when the parameter to be measured and by relation thecapacitance of capacitor 1802 is substantially constant. The processmeasures the capacitance 1802 a number of times equal to thepredetermined count. Variations in the measurement can be averaged outby having a large predetermined count. The process also allows for verysmall changes in capacitance to be measured very accurately. Theaccuracy of the measurement can be increased by raising thepredetermined count of the measurement cycles. In one embodiment, themeasured capacitance is an average determined by the measured elapsedtime and the predetermined count of measurement cycles. The measuredcapacitance can be translated to the parameter being measured such as aforce, pressure, or load. Data register 1814 can be configured to storethe parameter measurement or a number corresponding to the parametermeasurement.

FIG. 20 discloses a method 2000 for measuring a force, pressure, orload. The method description relates to and can reference FIGS. 1, 4, 6,8, 12, 13, and 19. The example disclosed herein uses a prostheticcomponent implementation but method 2000 can be practiced in any othersuitable system or device. The steps of method 2000 are not limited tothe order disclosed. Moreover, method 2000 can also have a greaternumber of steps or a fewer number of steps than shown.

At a step 2002, a force, pressure, or load is applied to a capacitor.Changes in the force, pressure, or load produce a corresponding changein a capacitance of the capacitor. At a step 2004, a repeating signal isgenerated. A time period of a single waveform of the repeating signal isa measurement cycle. The time period of the measurement cyclecorresponds to the capacitance of the capacitor. At a step 2006, thewaveform or signal is repeated a predetermined number of times. Ameasurement sequence comprises the repeated waveform for thepredetermined number of times. At a step 2008, an elapsed time of themeasurement sequence is measured. The elapsed time is the time requiredto generate the predetermined number of waveforms. At a step 2010, theforce, pressure, or load is maintained during the measurement sequence.In general, the force, pressure, or load coupled to the capacitor shouldbe constant during the measurement sequence. At a step 2012, themeasured elapsed time is correlated to the force, pressure, or loadmeasurement. Typically, a measurement range is known for the force,pressure, or load being applied to the capacitor. The capacitor orcapacitor type being used can be characterized using known force,pressure, and loads throughout the measurement range prior to use. Thus,a correlation between capacitance and force, pressure, or load is known.For example, the relationship between capacitance and force, pressure,or load can be stored in a look up table or by a mathematicalexpression. In one embodiment, the capacitor responds approximatelylinear throughout the measurement range. The average capacitance of thecapacitor can be calculated using the measured elapsed time to generatethe predetermined number of waveforms during the measurement sequence.The force, pressure, or load can then be determined from the previouscharacterization. Further refinement can be achieved by usingcalibration techniques during final testing of the capacitor. Thecalibration data on the capacitor can be used in the calculation of theforce, pressure, or load to further reduce measurement error. At a step2014, the predetermined number of waveforms can be increased to raisemeasurement accuracy. The measurement resolution can be increased bythis technique if the force, pressure, or load is substantially constantover the increased number of predetermined number waveforms. Moreover,the resolution supports measurement where the capacitance changes arerelatively small over the force, pressure, or load measurement range.

FIG. 21 illustrates a capacitor 2100 in accordance with an exampleembodiment. In general, a sensor for use in a medical environment isaccurate, reliable, low cost, and have a form factor suitable for theapplication. Sensors that produce an electrical signal require a wiredor wireless interconnect to electronic circuitry to receive, analyze,and provide the measurement data. Capacitor 2100 meets the above listedrequirements. Capacitor 2100 can be used in medical devices, tools, andequipment for measurement of different medical parameters. In theexample, capacitor 2100 can be integrated into devices, tools,equipment, and prosthetic components for measuring parameters of themuscular-skeletal system. Capacitor 2100 is suitable for intra-operativeand implantable prosthetic components that support installation andlong-term measurement of the installed structures.

Capacitor 2100 comprises a dielectric layer 2102, a dielectric layer2104, and a dielectric layer 2106. Capacitor 2100 comprises more thantwo capacitors in series mechanically. In one embodiment, capacitor 2100comprises 3 capacitors in mechanical series. Referring briefly to FIG.22, capacitor 2100 of FIG. 21 comprises capacitors 2206, 2204, and 2208.Capacitors 2206, 2204, and 2208 are coupled mechanically in series. Acompressive force, pressure, or load 2202 is applied to the seriescoupled capacitors 2206, 2204, and 2208. Referring back to FIG. 21, afirst capacitor comprises a conductive region 2108, dielectric layer2102, and conductive region 2110. The first capacitor corresponds tocapacitor 2204 of FIG. 22. Conductive regions 2108 and 2110 have apredetermined area such that the predetermined area, dielectric constantof dielectric layer 2102, and the thickness of dielectric layer 2102determine the capacitance of capacitor 2204. In one embodiment,conductive layer 2108 overlies, has substantially equal area, and isaligned to conductive layer 2110.

A second capacitor comprises conductive region 2108, dielectric layer2104, and a conductive region 2112. The second capacitor corresponds tocapacitor 2206 of FIG. 22. In one embodiment, conductive region 2112overlies, has approximately equal area, and is aligned to conductiveregion 2108. A load pad 2114 is formed overlying conductive region 2112.Load pad 2114 protects and prevents damage to conductive layer 2112 dueto a force, pressure or load applied to capacitor 2100.

A third capacitor comprises conductive region 2110, dielectric layer2106, and a conductive layer 2116. The third capacitor corresponds tocapacitor 2208 of FIG. 22. In one embodiment, conductive region 2116overlies, has approximately equal area, and is aligned to conductiveregion 2110. A load pad 2118 is formed overlying conductive region 2116.Load pad 2118 protects and prevents damage to conductive layer 2116 dueto a force, pressure or load applied to capacitor 2100. In general, loadpads 2114 and 2118 comprise a non-compressible material. Load pads 2114and 2218 can comprise metal, composite material, or a polymer.

Capacitor 2100 couples to electronic circuitry as disclosed in FIG. 18.Capacitor 2100 can comprise more than one capacitor in parallel. In oneembodiment, conductive regions 2108 and 2110 can be coupled in common.In the example, conductive regions 2108 and 2110 are coupled in commonby conductive via 2120. Conductive regions 2112 and 2116 are alsocoupled in common or to a common voltage potential. In one embodiment,conductive regions 2112 and 2116 are coupled to ground forming a shield.Referring briefly to FIG. 23, capacitor 2100 comprises capacitors 2206and 2208. Capacitors 2206 and 2208 are coupled electrically in parallelhaving a terminal coupled to ground and a terminal comprising conductiveregions 2108 and 2110 coupled in common. Capacitor 2204 is not shown inthe electrical equivalent circuit of capacitor 2100 because theconductive regions of capacitor 2204 are shorted together. Referringback to FIG. 21, capacitor 2206 and capacitor 2208 can be formed havingsubstantially equal capacitance. Thus, capacitor 2100 comprises morethan one capacitor that are mechanically in series and comprises morethan one capacitor that are coupled electrically in parallel.

In the example, capacitor 2100 can be used as a force, pressure, or loadsensor for the muscular-skeletal system. Capacitor 2100 can beintegrated into a prosthetic component to measure the force, pressure,or load applied by the muscular-skeletal system. The measurement hassupports the installation of prosthetic components and can be used forlong-term data collection on the implanted system. The size and shape ofcapacitor 2100 is beneficial to biological sensing applications. Theform factor of capacitor 2100 can be made very small. Moreover,capacitor 2100 can be made very thin which supports integration andplacement in regions of the body that could not be achieved withconventional sensors. A thickness of less 2.5 millimeters and typicallyless than 1 millimeter for capacitor 2100 can be manufactured.

In one embodiment, a multi-layered interconnect can be used to formcapacitor 2100. Multi-layer interconnect comprises alternatingconductive layers and dielectric layers. The conductive layers can bepatterned to form conductive regions and interconnect. Applying a force,pressure, or load to multi-layer interconnect can deform the dielectriclayers. It has been found that for small deformations the dielectriclayers of interconnect will rebound elastically when the stimulus isremoved. Deformation of the dielectric layer changes the dielectricthickness of capacitor 2100 and the capacitance value thereof. System1800 of FIG. 18 supports high resolution of small changes in capacitancethat makes the use of capacitor 2100 viable.

In general, the dielectric material for the interconnect can comprise apolymer, polyester, an aramid, an adhesive, silicon, glass, or compositematerial. Capacitor 2100 includes at least one dielectric layercomprising polyimide. In one example, dielectric layers 2102, 2104, and2106 comprise polyimide. Alternatively, layer 2102 can be an adhesivethat couples capacitors 2206 and 2208 together. Under testing, polyimidehas been shown to compress elastically under load values typical forprosthetic component load measurement. In general, capacitor 2100compresses less than 20% of thickness of each capacitor to maintainoperation in an elastic region of the dielectric. In one embodiment, thedielectric of capacitor 2100 is compressed less than 10% of thedielectric thickness over the operating range. For example, thepolyimide layer can be approximately 0.0254 millimeters thick.Compression of the polyimide can be less than 0.0022 millimeters overthe entire load measurement range for a prosthetic knee application. Theinterconnect can be flexible allowing placement on non-planar regions.Moreover, capacitor 2100 can be conformal to different surface shapes ifrequired. Alternatively, capacitor 2100 can be formed as a compressiblestructure that does not flex or conform.

As mentioned previously, capacitor 2100 is coupled to electroniccircuitry such as that disclosed in FIG. 18. Using interconnect to formcapacitor 2100 provides the further benefit of being able to integratecapacitor 2100 with the interconnect that couples to the electroniccircuitry. This eliminates a connection between the sensor and theinterconnect as they are formed as a single structure. The integratedcapacitor and interconnect also increases sensor reliability, lowerscost, and simplifies assembly.

Referring briefly to FIG. 24, a top view illustrates conductive region2112 formed overlying dielectric layer 2104. In general, the force,pressure, or load is applied uniformly on the conductive regions of thesensor capacitor. The load pad can support the distribution of theforce, pressure, or load across the entire conductive region. The areaof the conductive region is of sufficient size to maintain elasticcompression of the dielectric material over the entire force, pressure,or load range of the application. The area of the conductive regions canbe increased to reduce the force, pressure, or load per unit areathereby lowering dielectric compression over the measurement range forimproved reliability. In the knee prosthetic component example,conductive region 2112 can have a circular shape. The area of conductiveregion 2112 is a function of the force, pressure, or load range beingmeasured. The diameter of conductive region 2112 is approximately 2.0millimeters for a sensor for a knee application. The dashed lineindicates a periphery of conductive region 2108 that underliesconductive region 2112. In the example, conductive region 2108 has adiameter of approximately 2.2 millimeters. More than one of the sensorscan fit within a prosthetic component of the knee. An interconnect 2124is coupled to conductive region 2112. Interconnect 2124 can be formed onthe same layer as conductive region 2112. Referring back to FIG. 21,conductive region 2116 can have a similar circular shape as conductiveregion 2112. The diameter of conductive region 2116 is approximately 2.0millimeters for a sensor for a knee application. The conductive region2110 that overlies conductive region 2112 is approximately 2.2millimeters in diameter. An interconnect 2126 can be formed overlyingthe polyimide layer 2106 and couple to conductive region 2116.

In the example, a force, pressure, or load is applied by themuscular-skeletal system to load pads 2114 and 2118. The force,pressure, or load compresses capacitors 2206, 2204, and 2208

that are mechanically in series that comprise capacitor 2100. Dielectriclayers 2202, 2204, and 2206 compress under the force, pressure, or load.The plates of capacitor 2204 are coupled in common and do not contributeto a capacitance of capacitor 2100. The structure of capacitor 2100minimizes the effect of parasitic capacitance. Conductive regions 2112and 2116 are coupled to ground. Conductive regions 2112 and 2116respectively overlie and underlie conductive regions 2108 and 2110thereby acting as a ground shield. The shield minimizes or blocksexternal capacitive interaction that could occur with conductive regions2112 and 2116 that can effect measurement accuracy.

Referring briefly to FIG. 25, a cross-sectional view of interconnect2122, 2124, and 2126 in an example embodiment is provided. As describedhereinabove, conductive regions 2108 and 2110 are coupled in common byvia 2120. An interconnect 2122 couples to conductive regions 2108 and2110. Interconnect 2122, 2124, and 2126 can couple capacitor 2100 tosystem 1800 of FIG. 18. Interconnect 2124 and 2126 are coupled toground. Interconnect 2124 and 2126 overlie and underlie interconnect2122 thereby acting as a shield. In one embodiment, interconnect 2122has a width less than interconnects 2124 and 2126. Interconnects 2124and 2126 shield and block potential capacitive interaction withinterconnect 2122 as it is routed and coupled to system 1800 of FIG. 18.

Referring back to FIG. 21, parasitic capacitance related to capacitor2100 remains substantially constant throughout the parameter measurementrange. A first parasitic capacitance comprises interconnect 2124,dielectric layer 2104, and interconnect 2122. A second parasiticcapacitance comprises interconnect 2126, dielectric layer 2106, andinterconnect 2122. The first and second parasitic capacitances addtogether to increase the capacitance of capacitor 2100. The force,pressure, or load is not applied to first and second parasiticcapacitances thereby remaining constant during measurement. Thus, thechange in capacitance of capacitor 2100 can be measured by system 1800over the force, pressure, or load range using the method disclosedherein with secondary affects due to changes in parasitic capacitancebeing minimized.

FIG. 26 discloses a method 2600 for measuring a force, pressure, orload. The method description relates to and can reference FIGS. 1, 4, 6,8, 12, 13, 19, and 21-25. The steps of method 2600 are not limited tothe order disclosed. Moreover, method 2600 can also have a greaternumber of steps or a fewer number of steps than shown. At a step 2602,more than one capacitor in series is compressed. A sensor capacitor cancomprise more than one capacitor coupled in series. The force, pressure,or load is applied across the series coupled capacitors. At a step 2604,a capacitance of more than one capacitor in parallel is measured. Thesensor capacitor can comprise more than one capacitor electricallycoupled in parallel.

At a step 2606, a repeating signal is generated having a measurementcycle corresponding to capacitance of the more than one capacitor inparallel. In one embodiment, the more than one capacitor in parallel iscoupled to a signal generator circuit. The signal generator circuitcoupled to the more than one capacitor in parallel is configured tooscillate. The repeating signal comprises a repeating measurement cycle.A time period of each measurement cycle generated by the signalgenerator corresponds to the capacitance of the more than one capacitorin parallel.

At a step 2608, an elapsed time is measured of the repeating signal. Inone embodiment, the repeating signal is repeated a predetermined numberof times. In other words, the measurement cycle is repeated thepredetermined number of times and the elapsed time of the predeterminednumber of measurement cycles is measured. At a step 2610, the elapsedtime is correlated to the capacitance of the more than one capacitor inparallel. As disclosed herein, the capacitance of the more than onecapacitor in parallel corresponds to the applied force, pressure, orload. Measuring a large number of measurement cycles while the appliedforce, pressure, or load is substantially constant supports an accuratecorrelation between capacitance and the force, pressure, or load.

FIG. 27 illustrates a medical device having a plurality of sensors inaccordance with an example embodiment. In general, embodiments of theinvention are broadly directed to the measurement of physicalparameters. The medical device includes an electro-mechanical systemthat is configured to measure medical parameters and in the examplerelated to the measurement of the muscular-skeletal system. Manyphysical parameters of interest within physical systems or bodies arecurrently not measured due to size, cost, time, or measurementprecision. For example, joint implants such as knee, hip, spine,shoulder, and ankle implants would benefit substantially from in-situmeasurements taken during surgery to aid the surgeon in the installationand fine-tuning of a prosthetic system. Measurements can supplement thesubjective feedback of the surgeon to ensure optimal installation.Permanent sensors in the final prosthetic components can provideperiodic data related to the status of the implant in use. Datacollected intra-operatively and long term can be used to determineparameter ranges for surgical installation and to improve futureprosthetic components.

The physical parameter or parameters of interest can include, but arenot limited to, measurement of load, force, pressure, position,displacement, density, viscosity, pH, spurious accelerations, andlocalized temperature. Often, a measured parameter is used inconjunction with another measured parameter to make a qualitativeassessment. In joint reconstruction, portions of the muscular-skeletalsystem are prepared to receive prosthetic components. Preparationincludes bone cuts or bone shaping to mate with one or more prosthesis.Parameters can be evaluated relative to orientation, alignment,direction, displacement, or position as well as movement, rotation, oracceleration along an axis or combination of axes by wireless sensingmodules or devices positioned on or within a body, instrument,appliance, vehicle, equipment, or other physical system.

In the present invention parameters are measured with an integratedwireless sensing module or device comprising an i) encapsulatingstructure that supports sensors and contacting surfaces and ii) anelectronic assemblage that integrates a power supply, sensing elements,an accelerometer, antennas, electronic circuitry that controls andprocesses a measurement sequence, and wireless communication circuitry.The wireless sensing module or device can be positioned on or within, orengaged with, or attached or affixed to or within, a wide range ofphysical systems including, but not limited to instruments, equipment,devices, appliances, vehicles, equipment, or other physical systems aswell as animal and human bodies, for sensing and communicatingparameters of interest in real time.

Sensors are disclosed that can indirectly measure the parameter such asa capacitor having a capacitance that varies with the parameter. Thecapacitance or related factor (e.g. time) is measured and then convertedto the parameter. The measurement system has a form factor, power usage,and material that is compatible with human body dynamics. The physicalparameter or parameters of interest can include, but are not limited to,measurement of load, force, pressure, displacement, density, viscosity,pH, distance, volume, pain, infection, spurious acceleration, andlocalized temperature to name a few. These parameters can be evaluatedby sensor measurement, alignment, direction, or position as well asmovement, rotation, or acceleration along an axis or combination of axesby wireless sensing modules or devices positioned on or within a body,instrument, appliance, vehicle, equipment, or other physical system.

In the example, an insert 2700 illustrates a device having a medicalsensor for measuring a parameter of the muscular-skeletal system.Prosthetic insert 2700 is a component of a joint replacement system thatallows articulation of the muscular-skeletal system. The prostheticinsert 2700 is a wear component of the joint replacement system. Theprosthetic insert 2700 has one or more articular surfaces that allowjoint articulation. In a joint replacement, a prosthetic component has asurface that couples to the articular surface of the insert 2700. Thearticular surface is low friction and can absorb loading that occursnaturally based on situation or position. The contact area betweensurfaces of the articulating joint can vary over the range of motion.The articular surface of insert 2700 will wear over time due to frictionproduced by the prosthetic component surface contacting the articularsurface during movement of the joint. Ligaments, muscle, and tendonshold the joint together and motivate the joint throughout the range ofmotion.

Insert 2700 is an active device having a power source 2702, electroniccircuitry 2704, load pads 2722, transmit capability, and sensors withinthe body of the prosthetic component. Electronic circuitry 2704 includesthe circuitry of FIG. 18 and FIG. 19. In the example, sensors underlieload pads 2722. The sensors are capacitors formed in an interconnect2718 that couples to electronic circuitry 2704. Interconnect 2718 can beflexible and conformal to non-planar shapes. In one embodiment, insert2700 is used intra-operatively to measure parameters of themuscular-skeletal system to aid in the installation of one or moreprosthetic components. As will be disclosed hereinbelow, operation ofinsert 2700 is shown as a knee insert to illustrate operation andmeasurement of a parameter such as load and balance. Referring brieflyto FIG. 1, a typical knee joint replacement system comprises an insert,femoral prosthetic component 104, and tibial prosthetic component 106.Although housed in the insert, sensor capacitors can also be housedwithin or coupled to femoral prosthetic component 104 or tibialprosthetic component 106. Referring back to FIG. 27, insert 2700 can beadapted for use in other prosthetic joints having articular surfacessuch as the hip, spine, shoulder, ankle, and others. Alternatively,insert 2700 can be a permanent active device that can be used to takeparameter measurements over the life of the implant. The sensing systemis not limited to the prosthetic component example. The system can alsobe implemented in medical tools, devices, and equipment.

Insert 2700 is substantially equal in dimensions to a passive finalprosthetic insert. The substantially equal dimensions correspond to asize and shape that allow insert 2700 to fit substantially equal to thepassive final prosthetic insert in a tibial prosthetic component. In theintra-operative example, the measured load and balance using insert 2700as a trial insert would be substantially equal to the loading andbalance seen by a final passive insert under equal conditions. It shouldbe noted that insert 2700 for intra-operative measurement could bedissimilar in shape or have missing features that do not benefit thetrial during operation. Insert 2700 should be positionally stablethroughout the range of motion equal to that of the final insert.

The exterior structure of insert 2700 comprises two components. In theembodiment shown, insert 2700 comprises a support structure 2706 and asupport structure 2708. Support structures 2706 and 2708 have majorsupport surfaces that are loaded by the muscular-skeletal system. Aspreviously mentioned, insert 2700 is shown as a knee insert toillustrate general concepts and is not limited to this configuration.Support structure 2706 has an articular surface 2710 and an articularsurface 2712. Condyles of a femoral prosthetic component articulate withsurfaces 2710 and 2712. Loading on the prosthetic knee joint isdistributed over a contact area of the articular surfaces 2710 and 2712.Support structure 2708 has a load-bearing surface 2724. The load-bearingsurface 2724 couples to the tibial prosthetic component. The loading onload-bearing surface 2724 is much lower than that applied to thearticular surfaces due to the larger surface area for distributing aforce, pressure, or load.

A region 2714 of the support structure 2706 is unloaded or is lightlyloaded over the range of motion. Region 2714 is located between thearticular surfaces 2710 and 2712. It should be noted that there is aminimum area of contact on articular surfaces 2710 and 2712 to minimizewear while maintaining joint performance. The contact location andcontact area size can vary depending on the position of themuscular-skeletal system. Problems may occur if the contact area fallsoutside a predetermined area range within articular surfaces 2710 and2712 over the range of motion. In one embodiment, the location where theload is applied on articular surfaces 2710 and 2712 can be determined bythe sensing system. This is beneficial because the surgeon now hasquantitative information where the loading is applied. The surgeon canthen make adjustments that move the location of the applied load withinthe predetermined area using real-time feedback from the sensing systemto track the result of each correction.

The support structure 2708 can be formed to support the sensors andelectronic circuitry 2704 that measure loading on each articular surfaceof insert 2700. A load plate 2716 underlies articular surface 2710.Similarly, a load plate 2720 underlies articular surface 2712.Interconnect 2718 underlies load plate 2720. Capacitor sensors underlieload pads 2722 in the vertices of the triangular shaped interconnect2718 in support structure 2708. In one embodiment, the capacitor sensorsare formed in the interconnect 2718. Interconnect 2718 couples thesensors to electronic circuitry 2704. A shield is formed in interconnect2718 that minimizes parasitic capacitance and coupling to ensureaccuracy over the measurement range. Load plate 2720 couples to thecapacitor sensors through load pads 2722. Load plate 2720 distributesthe load applied to articular surface 2712 to the capacitor sensors atpredetermined locations within insert 2700. The measurements from thethree sensors underlying articular surface 2712 can be used to determinethe location of the applied load to insert 2700. Load plate 2716operates similarly underlying articular surface 2710. Although thesurface of load plates 2716 and 2720 as illustrated are planar they canbe non-planar with the sensors conforming to the non-planar surface.Similarly, the capacitor sensors can formed having a non-planar shape.

A force, pressure, or load applied by the muscular-skeletal system iscoupled to the articular surfaces 2710 and 2712 of prosthetic componentinsert 2700, which respectively couples to plates 2716 and 2720. In oneembodiment, each capacitor elastically compresses due to the force,pressure, or load. Electronic circuitry 2704 is operatively coupled tothe capacitor sensors underlying load plates 2716 and 2720. A signal isgenerated that corresponds to the capacitance of the capacitor beingmeasured. The signal is repeated a predetermined number of times or fora predetermined count. The elapsed time of the predetermined count ismeasured. The elapsed time corresponds to the capacitance of thecapacitor. The relationship between capacitance and force, pressure, orload is known and used to determine the measurement value. Furthermore,the measurement data can be processed and transmitted to a receiverexternal to insert 2700 for display and analysis.

In one embodiment, the physical location of the sensors and electroniccircuitry 2704 is housed in insert 2700 thereby protecting the activecomponents from an external environment. Electronic circuitry 2704 canbe located between articular surfaces 2710 and 2712 underlying region2714 of support structure 2700. A cavity for housing the electroniccircuitry 2704 can underlie region 2714. Support structure 2708 has asurface within the cavity having retaining features extending therefromto locate and retain electronic circuitry 2704 within the cavity. Region2714 is an unloaded or a lightly loaded region of insert 2700 therebyreducing the potential of damaging the electronic circuitry 2704 due toa high compressive force during surgery or as the joint is used by thepatient. In one embodiment, a temporary power source such as a battery,capacitor, inductor, or other storage medium is located within insert2700 to power the sensors and electronic circuitry 2704.

Support structure 2706 attaches to support structure 2708 to form aninsert casing or housing. In one embodiment, internal surfaces ofsupport structures 2706 and 2708 mate together. Moreover, the internalsurfaces of support structures 2706 and 2708 can have cavities orextrusions to house and retain components of the sensing system.Externally, support structures 2706 and 2708 provide load bearing andarticular surfaces that interface to the other prosthetic components ofthe joint. The load-bearing surface 2724 of support structure 2708couples to the tibial prosthetic component. Load-bearing surface 2724can have one or more features or a shape that supports coupling to thetibial prosthetic component.

The support structures 2706 and 2708 can be temporarily or permanentlycoupled, attached, or fastened together. As shown, insert 2700 can betaken apart to separate support structures 2706 and 2708. A seal can belocated peripherally on an interior surface of support structure 2708.In one embodiment, the seal can be an O-ring that comprises a compliantand compressible material. The O-ring compresses and forms a sealagainst the interior surface of support structures 2706 and 2708 whenattached together. Support structures 2706 and 2708 form a housingwhereby the cavities or recesses within a boundary of the seal areisolated from an external environment. In one embodiment supportstructures 2706 and 2708 are coupled together when the O-ring iscompressed sufficiently to interlock fastening elements. Supportstructures 2706 and 2708 are held together by the fastening elementsunder force or pressure provided by the O-ring or other means such as aspring.

In one embodiment, support structure 2700 comprises material commonlyused for passive inserts. For example, ultra high molecular weightpolyethylene can be used. The material can be molded, formed, ormachined to provide the appropriate support and articular surfacethickness for a final insert. Alternatively, support structures 2706 and2708 can be made of metal, plastic, or polymer material of sufficientstrength for a trial application. In an intra-operative example, supportstructures 2706 and 2708 can be formed of polycarbonate. It should benoted that the long-term wear of the articular surfaces is a lesserissue for the short duration of the joint installation. The joint movessimilarly to a final insert when moved throughout the range of motionwith a polycarbonate articular surface. Support structures 2706 and 2708can be a formed as a composite where a bearing material such as ultrahigh molecular weight polyethylene is part of the composite materialthat allows the sensing system to be used both intra-operatively and asa final insert.

FIG. 28 illustrates one or more prosthetic components having sensorscoupled to and conforming with non-planar surfaces in accordance with anexample embodiment. Hip joint prosthetic components are used as anexample to illustrate non-planar sensors. The hip joint prosthesiscomprises an acetabular cup 2806, an insert 2808, and a femoralprosthetic component 2810. The acetabular cup 2806 couples to a pelvis.Cup 2806 can be cemented to pelvis 2802 thereby fastening the prostheticcomponent in a permanent spatial orientation for receiving femoralprosthetic component 2810. Insert 2808 is inserted into acetabular cup2806 having an exposed articular surface. A femoral head of femoralprosthetic component 2810 can be placed into insert 2808. Insert 2808retains the femoral head. The articular surface of insert 2808 couplesto the femoral head of femoral prosthetic component 2810 allowingrotation of the joint. The loading is distributed over an area of thearticular surface of insert 2808 that varies depending on the legposition. A shaft of femoral prosthetic component 2810 is coupled to afemur 2804. Cement can be used to fasten the shaft of femoral prostheticcomponent 2810 to femur 2804. Tissue such as tendons, ligaments, andmuscle couple to pelvis 2802 and femur 2804 to retain and supportmovement of the hip joint. The sensors and electronic circuitrydisclosed herein are not limited to prosthetic hip components and can beapplied similarly to other parts of the anatomy including but notlimited to the muscular-skeletal system, bone, organs, skull, knee,shoulder, spine, ankle, elbow, hands, and feet.

In one embodiment, femoral prosthetic component 2810 can houseelectronic circuitry 2812 thereby protecting the active components froman external environment. The electronic circuitry 2812 can include thecircuitry disclosed in FIG. 18 and FIG. 19 to measure capacitance of acapacitor sensor. The electronic circuitry 2812 can further include apower source, power management circuitry, conversion circuitry, digitallogic, processors, multiple input/output circuitry, and communicationcircuitry. The electronic circuitry 2812 can be a module having a formfactor that can fit within a prosthetic component. Similarly, electroniccircuitry 2812 can be integrated into a tool, device, or equipment.Alternatively, electronic circuitry 2812 can be a separate componentthat couples through a wired or wireless connection to sensors.

The femoral head of the prosthetic component 2810 is spherical in shape.Capacitors 2814 are sensors that conform and couple to the curvedsurface of the femoral head. In first embodiment, capacitors 2814 canunderlie an external surface of the femoral head. A force, pressure, orload applied to the femoral head couples to and can elastically compresscapacitors 2814. Capacitors 2814 and electronic circuitry 2812 areprotected from an external environment such that the prostheticcomponent is suitable for long term monitoring of the joint. In a secondembodiment, capacitors 2814 can be exposed on portions of the surfaceconforming to a spherical shape of the femoral head. In a thirdembodiment, capacitors 2814 can be formed having the non-planar shape.Capacitors 2814 can be in a trial prosthetic component that is disposedof after a single use. As disclosed herein, capacitors 2814 can beformed in interconnect as disclosed in FIGS. 21-25. The interconnect canbe flexible and can conform to non-planar surfaces. In the example,capacitors 2814 are formed in interconnect that couples to electroniccircuitry 2812 to receive and process measurement data. The interconnectand more specifically capacitors 2814 are positioned within and coupledto the spherical femoral head surface whereby force, pressure, or loadscan be measured at predetermined locations. Thus, the sensor system canbe housed entirely within a prosthetic component. Similarly, the sensorscan be placed on, within or between acetabular cup 2806 and insert 2808.As an example, capacitors 2816 are shown placed between acetabular cup2806 and insert 2808. Capacitors 2816 can also underlie or comprise aportion of the articular surface of insert 2808. Similarly, capacitors2816 can underlie or comprise a portion of the curved surface ofacetabular cup 2806. Capacitors 2816 can be configured to measure force,pressure, or load applied to different regions of the articular surfaceof insert 2808. Electronic circuitry coupled to capacitors 2816 can bein proximity to or housed in acetabular cup 2806, insert 2808. Force,pressure, or load measurements on bone can be supported by the system.Capacitors 2822 can be embedded in bone such as pelvis 2802 to measureforces applied thereto.

In the example, capacitors 2814 are located at predetermined locationsof the femoral head of femoral prosthetic component 2810. Thecapacitance of capacitors 2814 relate to the force, pressure, or loadapplied to the femoral head by the muscular-skeletal system therebyproviding measurement data at the different locations of the femoralhead. In one embodiment, measurement data from capacitors 2814 can bewirelessly transmitted to a remote system 2818 in real-time. Remotesystem 2818 includes a display 2820 configured to display themeasurement data. Remote system 2818 can be a computer that furtherprocesses the measurement data. The measurement data can be provided inan audible, visual, or haptic format that allows the user to rapidlyassess the information. Rotating and moving the leg over the range ofmotion can provide quantitative data on how the loading varies over therange of motion of the hip joint for the installation. The leg movementcouples capacitors 2814 to different areas of the articular surface ofinsert 2808. Capacitors 2814 move in an arc when the leg is moved in aconstant plane. The measurements data can indicate variations in loadingthat can require modification to the joint installation. Theinstallation can be done in workflow steps that are supported by remotesystem 2818. Moreover, clinical evidence from quantitative measurementsover a statistically significant number of patients as target values orranges for an optimal fit. The surgeon can further fine-tune theinstallation based on the actual measured quantitative data andsubjective feedback from the patient installation.

FIG. 29 illustrates a tool having one or more shielded sensors coupledto a non-planar surface in accordance with an example embodiment. Areamer 2902 is used as an example of a medical device, tool, equipment,or component having one or more sensors. Reamer 2902 can be used in ahip prosthetic joint replacement surgery for removing bone in a pelvis2908 to accept a prosthetic component such as an acetabular cup. Reamer2902 has spherical shaped surface 2904 having cutting blades orabrasives for removing bone in an acetabular region 2910 to form aspherical shaped bone region. The cutting head of reamer 2902 is sizedto cut acetabular region 2910 region substantial equal in dimensions tothe acetabular cup to be fitted therein.

In one embodiment, more than one sensor can be coupled to the cuttinghead of reamer 2902. In a non-limiting example, the sensors can be usedto measure a force, pressure, or load. More specifically, the sensorscan be positioned corresponding to locations on surface 2904 of thecutting head. The sensors are coupled to surface 2904 but are internalto the cutting head of reamer 2902. The force, pressure, or load iscoupled from surface 2904 to the sensors. The sensors providequantitative data on the force, pressure, or load applied to thedifferent locations of surface 2904. The quantitative data can be usedas feedback to the material removal process for optimal fit of theacetabular cup. For example, placing too much force in one direction canresult in too much material being removed in a location therebyaffecting the shape of the bone cut.

Capacitors 2906 are an example of sensors for measuring a force,pressure, or load. Capacitors 2906 are elastically compressible over themeasurable range of reamer 2902. More specifically, the dielectricmaterial comprising capacitors 2906 compresses under an applied force,pressure, or load. The capacitance of a capacitor increases as thedielectric material decreases in thickness due to the force, pressure,or load. Conversely, the dielectric material increases in thickness asthe force, pressure, or load applied to the capacitor is reduced therebydecreasing a capacitance value. Capacitors 2906 are coupled to differentlocations of surface 2904 of the cutting head of reamer 2902. Thecapacitors 2906 are distributed across surface 2904 to provide force,pressure, or load magnitudes and differential force, pressure, or loadmagnitudes for different surface regions during a material removalprocess. The surface regions being measured by capacitors 2906 willchange with the trajectory of reamer 2902. The measurement data can beused to support a bone reaming process for optimal prosthetic componentfit.

In one embodiment, capacitors 2906 are formed within an interconnect asdisclosed in FIGS. 21-25. The interconnect can include one or moredielectric layers or substrates comprising polyimide. The polyimidelayers are flexible, can conform to a non-planar surface, or be formedhaving a predetermined shape. Capacitors 2906 include one or moreshields to reduce capacitive coupling to the device. A shield can becoupled to ground and be physically between a conductive region ofcapacitors 2906 and an external environment of the interconnect. Theshield can be a conductive region of the capacitor. In one embodiment, afirst shield is formed overlying a conductive region of a capacitor anda second shield is formed underlying the conductive region of thecapacitor. The shield minimizes parasitic capacitances that can change acapacitance value of capacitors 2906.

Interconnect can be formed on the one or more polyimide layers thatcouples to the conductive regions of capacitors 2906. The interconnectcan couple capacitors 2906 to electronic circuitry (not shown) forgenerating a signal corresponding to a capacitance of each capacitor.Capacitors 2906 couple to surface 2904 of the cutting head of reamer2902. In the example, capacitors 2906 conform to a curved or non-planarsurface corresponding to a shape of surface 2904. In one embodiment, theinterconnect and capacitors 2906 are internal to the cutting headthereby isolated from an external environment. The interconnect couplesto electronic circuitry for measuring capacitance of capacitors 2906.The electronic circuitry can be housed in the cutting head or the handleof reamer 2902. The electronic circuitry can include a power source suchas a battery, inductive power source, super capacitor, or other storagemedium. As mentioned previously, the capacitance of capacitors 2906 canbe related to a force, pressure, or load applied thereto. In theexample, the electronic circuitry generates a signal for each capacitorof capacitors 2906 that relates to a capacitance value. The electroniccircuitry can include transmit and receive circuitry for sendingmeasurement data from capacitors 2906. In one embodiment, the measureddata is transmitted to a remote system 2818. Remote system 2818 caninclude a display 2820 for presenting the measurement data. Dataprocessing can be performed by remote system 2818 to convert themeasurement data to a force, pressure, or load. Trajectory data andforce, pressure, or load measurements can be provided in a visual formatthat allows rapid assessment of the information. Audible feedback can beprovided to supplement display 2820 when the user requires directviewing of an operational area. Remote system 2818 can analyze thequantitative measurement data and transmit information to reamer 2902that provides haptic or other types of feedback to the device thataffects trajectory or force, pressure, or load as directed by the user.Quantitative data provided by reamer 2902 is provided in real-timeallowing the user to see how the changes affect bone removal on pelvis2908 on display 2820.

FIG. 30 discloses a method 3000 for measuring a force, pressure, orload. The method description relates to and can reference FIGS. 1, 4, 6,8, 12, 13, 19, 21-25, and 27-29. The steps of method 3000 are notlimited to the order disclosed. Moreover, method 3000 can also have agreater number of steps or a fewer number of steps than shown. At a step3002, a force, pressure, or load is applied to a capacitor. Changes inthe force, pressure, or load produce a corresponding change in acapacitance of the capacitor. In one embodiment, the capacitor is formedon or in an interconnect. The dielectric material of the capacitor canbe elastically compressible. In a step 3004, at least one conductiveregion of the capacitor is shielded to reduce capacitive coupling. Inone embodiment, the shield can comprise a conductive region of thecapacitor that is a plate of the capacitor. Alternatively, the shieldcan be a separate structure. The shield can be grounded to minimizeparasitic capacitance or coupling to the capacitor. The shield can bebetween an external environment of the capacitor and the activeconductive region or plate of the capacitor being shielded. Furthermore,the shield reduces variable parasitic capacitance that can affectmeasurement accuracy. The grounded conductive region can be between theactive conductive region and the external environment. In a step 3006,interconnect coupling the capacitor to electronic circuitry is shieldedto further reduce capacitive coupling. The shield can be an interconnectof the capacitor. For example, a grounded interconnect can be placedbetween the interconnect carrying a signal and an external environmentto prevent capacitive coupling from circuitry in the externalenvironment. Alternatively, the shield can be a separate structure.Shielding for the capacitor and the interconnect supports themeasurement of very small capacitive values. The change in measuredcapacitance can be small in comparison to the total capacitance.Shielding prevents the total capacitance from changing thereby allowinga capacitance change of less than 10 picofarads to be measured.

Thus, a system is provided herein for measuring small capacitive valuesand small changes in capacitance. The system further supports a smallform factor, high reliability, measurement accuracy, and low cost.Capacitors for force, pressure, and load measurement can be formed ininterconnect used to couple the capacitors to electronic circuitry. Thecapacitors are operated within a substantially elastically compressibleregion of the dielectric material. Forming the capacitors in theinterconnect reduces system complexity, improves reliability, productconsistency, and reduces assembly steps.

A signal is generated corresponding to a capacitance of the capacitorunder a force, pressure, or load. The signal is repeated for apredetermined count. Measuring an elapsed time of a large number ofmeasurement cycles can be used to generate an average time period of ameasurement cycle when change in the parameter being measured occursslowly in relation to physiological changes such as occurs in themuscular-skeletal system. The measurement data can be analyzed toachieve accurate, repeatable, high precision and high-resolutionmeasurements. The system disclosed herein enables the setting of thelevel of precision or resolution of captured data to optimize trade-offsbetween measurement resolution versus frequency, including the bandwidthof the sensing and data processing operations, thus enabling a sensingmodule or device to operate at its optimal operating point withoutcompromising resolution of the measurements. This is achieved by theaccumulation of multiple cycles of excitation and transit time insteadof averaging transit time of multiple individual excitation and transitcycles. The result is accurate, repeatable, high precision andhigh-resolution measurements of parameters of interest in physicalsystems.

Measurement using elastically compressible capacitors enables highsensitivity and high signal-to-noise ratio. The time-based measurementsare largely insensitive to most sources of error that may influencevoltage or current driven sensing methods and devices. The resultingchanges in the transit time of operation correspond to frequency, whichcan be measured rapidly, and with high resolution. This achieves therequired measurement accuracy and precision thus capturing changes inthe physical parameters of interest and enabling analysis of theirdynamic and static behavior.

Furthermore, summing individual capacitive measurements before dividingto estimate the average measurement value data values produces superiorresults to averaging the same number of samples. The resolution of countdata collected from a digital counter is limited by the resolution ofthe least significant bit in the counter. Capturing a series of countsand averaging them does not produce greater precision than this leastsignificant bit that is the precision of a single count. Averaging doesreduce the randomness of the final estimate if there is random variationbetween individual measurements. Summing the counts of a large number ofmeasurement cycles to obtain a cumulative count then calculating theaverage over the entire measurement period improves the precision of themeasurement by interpolating the component of the measurement that isless than the least significant bit of the counter. The precision gainedby this procedure is on the order of the resolution of the leastsignificant bit of the counter divided by the number of measurementcycles summed.

The present invention is applicable to a wide range of medical andnonmedical applications including, but not limited to, frequencycompensation; control of, or alarms for, physical systems; or monitoringor measuring physical parameters of interest. The level of accuracy andrepeatability attainable in a highly compact sensing module or devicemay be applicable to many medical applications monitoring or measuringphysiological parameters throughout the human body including, notlimited to, bone density, movement, viscosity, and pressure of variousfluids, localized temperature, etc. with applications in the vascular,lymph, respiratory, digestive system, muscles, bones, and joints, othersoft tissue areas, and interstitial fluids.

While the present invention has been described with reference toparticular embodiments, those skilled in the art will recognize thatmany changes may be made thereto without departing from the spirit andscope of the present invention. Each of these embodiments and obviousvariations thereof is contemplated as falling within the spirit andscope of the invention.

What is claimed is:
 1. A medical sensor for measuring a parametercomprising: a polyimide layer; a first conductive region overlying thepolyimide layer; and a second conductive region underlying the polyimidelayer where the medical sensor is configured such that a change in theparameter produces a change in the polyimide layer.
 2. The sensor ofclaim 1 where the medical sensor is configured to receive a forcepressure or load.
 3. The sensor of claim 2 further including: a firstdielectric layer overlying the first conductive layer; and a firstshield region overlying the second dielectric layer.
 4. The sensor ofclaim 3 further including: a second dielectric layer underlying thesecond conductive region; and a second shield region underlying thethird dielectric layer.
 5. The sensor of claim 4 where the sensor isconfigured to compress the polyimide layer when a force, pressure, orload is applied to the sensor.
 6. The sensor of claim 5 where at leastone of the first and second dielectric layers comprise polyimide andwhere the polyimide layer and at least one of the first and seconddielectric layers compresses when a force, pressure, or load is appliedto the sensor.
 7. The sensor of claim 6 where the first and secondconductive regions are coupled in common.
 8. The sensor of claim 7 wherethe first and second shield regions are coupled in common.
 9. The sensorof claim 8 where a capacitance of the sensor varies with a force,pressure, or load applied thereto.
 10. The sensor of claim 1 where thesensor is configured to be coupled to a prosthetic component, tool,equipment, or device.
 11. The sensor of claim 1 where the sensor isconfigured to measure a force, pressure, or load applied by the muscularskeletal system.
 12. A compressible medical sensor for measuring aparameter comprising at least two capacitors mechanically in serieswhere at least one of the two capacitors includes a substantiallyelastically compressible dielectric material.
 13. The sensor of claim 12where the sensor is coupled to a prosthetic component.
 14. The sensor ofclaim 12 where the sensor is coupled to the muscular-skeletal system.15. The sensor of claim 12 where at least one of the two capacitorscomprises a dielectric of polyimide.
 16. The sensor of claim 12 where atleast one of a polymer, polyester, aramid, silicon, glass, or compositematerial is used as a dielectric in at least one of the two capacitors.17. The sensor of 12 further including a third capacitor where the threecapacitors are mechanically in series.
 18. The sensor of claim 12 wherethe sensor comprises: a first dielectric layer; a first conductiveregion overlying the dielectric layer; a second conductive regionunderlying the first dielectric layer; a second dielectric layeroverlying the first conductive region; a first shield region overlyingthe second dielectric layer; a third dielectric layer underlying thesecond conductive region; and a second shield region underlying thethird dielectric layer.
 19. A method of sensing comprising the steps of:compressing more than one capacitor in series; and measuring the morethan one capacitor in parallel.
 20. The method of sensing of claim 19further including the steps of: generating a repeating signal having ameasurement cycle corresponding to the capacitance of the more than onecapacitor in parallel; measuring an elapsed time of the repeating signalwhere the signal is repeated a predetermined number of times; andcorrelating the elapsed time to the capacitance of the more than onecapacitor in parallel.